War War is an organized and often prolonged conflict that is carried out by states or non - state actor s . It is generally characterised by extreme violence , social disruption and economic destruction . War should be understood as an actual , intentional and widespread armed conflict between political communities , and therefore is defined as a form of political violence or intervention . The set of techniques used by a group to carry out war is known as warfare . An absence of war is usually called peace . While some scholars see warfare as an inescapable and integral aspect of human nature , others argue that it is only inevitable under certain socio - cultural or ecological circumstances . For some the practice of war is not linked to any single type of political organization or society . Rather , as discussed by John Keegan in his " A History of Warfare " , war is a universal phenomenon whose form and scope is defined by the society that wages it . Another argument suggests that since there are human societies in which warfare does not exist , humans may not be naturally disposed for warfare , which emerges under particular circumstances . The deadliest war in history , in terms of the cumulative number of deaths since its start , is the Second World War , with 60 – 85 million deaths . Proportionally speaking , the most destructive war in modern history is the War of the Triple Alliance , which took the lives of over 60 % of Paraguay ' s population , according to Steven Pinker . In 2003 , Richard Smalley identified war as the sixth ( of ten ) biggest problems facing humanity for the next fifty years . Etymology . The English word " war " derives from the late Old English ( c . 1050 ) words " wyrre " and " werre " ; the Old French " werre " ; the Frankish " werra " ; and the Proto - Germanic " werso " . The denotation of " war " derives from the Old Saxon " werran " , Old High German " werran " , and the German " verwirren " : “ to confuse ” , “ to perplex ” , and “ to bring into confusion ” . Another posited derivation is from the Ancient Greek " barbaros " , the Old Persian " varhara " , and the Sanskrit " varvar " and " barbara " . In German , the equivalent is " Krieg " ; the equivalent Spanish , Portuguese , and Italian words for " war " is " guerra " , derived from the Germanic " werra " ( “ fight ” , “ tumult ” ) . Etymologic legend has it that the Romanic peoples adopted a foreign , Germanic word for " war " , to avoid using the Latin " bellum " , because , when sounded , it tended to merge with the sound of the word " bello " ( " beautiful " ) . The scholarly study of war is sometimes called polemology ( ) , from the Greek " polemos " , meaning " war " , and " - logy " , meaning " the study of " . Types . War , to become known as one , must entail some degree of confrontation using weapons and other military technology and equipment by armed forces employing military tactics and operational art within the broad military strategy subject to military logistics . War Studies by military theorists throughout military history have sought to identify the philosophy of war , and to reduce it to a military science . Modern military science considers several factors before a national defence policy is created to allow a war to commence : the environment in the area ( s ) of combat operations , the posture national forces will adopt on the commencement of a war , and the type of warfare troops will be engaged in . Behaviour and conduct . The behaviour of troops in warfare varies considerably , both individually and as units or armies . In some circumstances , troops may engage in genocide , war rape and ethnic cleansing . Commonly , however , the conduct of troops may be limited to posturing and sham attacks , leading to highly rule - bound and often largely symbolic combat in which casualties are much reduced from that which would be expected if soldiers were genuinely violent towards the enemy . Situations of deliberate dampening of hostilities occurred in World War I by some accounts , e . g . , a volley of gunfire being exchanged after a misplaced mortar hit the British line , after which a German soldier shouted an apology to British forces , effectively stopping a hostile exchange of gunfire . Other examples of non - aggression , also from World War I , are detailed in " Good - Bye to All That . " These include spontaneous ceasefires to rebuild defences and retrieve casualties , alongside behaviour such as refusing to shoot at enemy during ablutions and the taking of great risks ( described as 1 in 20 ) to retrieve enemy wounded from the battlefield . The most notable spontaneous ceasefire of World War I was the Christmas truce . The psychological separation between combatants , and the destructive power of modern weaponry , may act to override this effect and facilitate participation by combatants in the mass slaughter of combatants or civilians , such as in the bombing of Dresden in World War II . The unusual circumstances of warfare can incite apparently normal individuals to commit atrocities . History . Before the dawn of civilization , war likely consisted of small - scale raiding . One half of the people found in a Nubian cemetery dating to as early as 12 , 000 years ago had died of violence . Since the rise of the state some 5 , 000 years ago , military activity has occurred over much of the globe . The advent of gunpowder and the acceleration of technological advances led to modern warfare . According to Conway W . Henderson , " One source claims that 14 , 500 wars have taken place between 3500 BC and the late 20th century , costing 3 . 5 billion lives , leaving only 300 years of peace ( Beer 1981 : 20 ) . " In " War Before Civilization " , Lawrence H . Keeley , a professor at the University of Illinois , says that approximately 90 – 95 % of known societies throughout history engaged in at least occasional warfare , and many fought constantly . Keeley describes several styles of primitive combat such as small raids , large raids , and massacres . All of these forms of warfare were used by primitive societies , a finding supported by other researchers . Keeley explains that early war raids were not well organized , as the participants did not have any formal training . Scarcity of resources meant that defensive works were not a cost effective way to protect the society against enemy raids . William Rubinstein wrote that " Pre - literate societies , even those organised in a relatively advanced way , were renowned for their studied cruelty . . . ' archaeology yields evidence of prehistoric massacres more severe than any recounted in ethnography [ ie , after the coming of the Europeans ] ' . At Crow Creek , South Dakota , as noted , archaeologists found a mass grave of ' more than 500 men , women , and children who had been slaughtered , scalped , and mutilated during an attack on their village a century and a half before Columbus ' s arrival ( ca . AD 1325 ) ' " . It is problematic however to suggest that people in past societies are any more violent than people today . Martin and colleagues in their recent book have pulled together some of the foremost researchers studying violence in the past to show that though there may be events like massacres in the past , like today the use and manifestation of warfare varies greatly both within and between societies . In Western Europe , since the late 18th century , more than 150 conflicts and about 600 battles have taken place . During the 20th century , war resulted in a dramatic intensification of the pace of social changes , and was a crucial catalyst for the emergence of the Left as a force to be reckoned with . Recent rapid increases in the technologies of war , and therefore in its destructiveness ( see mutual assured destruction ) , have caused widespread public concern , and have in all probability forestalled , and may altogether prevent the outbreak of a nuclear World War III . At the end of each of the last two World Wars , concerted and popular efforts were made to come to a greater understanding of the underlying dynamics of war and to thereby hopefully reduce or even eliminate it all together . These efforts materialized in the forms of the League of Nations , and its successor , the United Nations . Shortly after World War II , as a token of support for this concept , most nations joined the United Nations . During this same post - war period , with the aim of further delegitimizing war as an acceptable and logical extension of foreign policy , most national governments also renamed their Ministries or Departments of War as their Ministries or Departments of Defense , for example , the former US Department of War was renamed as the US Department of Defense . In 1947 , in view of the rapidly increasingly destructive consequences of modern warfare , and with a particular concern for the consequences and costs of the newly developed atom bomb , Albert Einstein famously stated , " I know not with what weapons World War III will be fought , but World War IV will be fought with sticks and stones . " Mao Zedong urged the socialist camp not to fear nuclear war with the United States since , even if " half of mankind died , the other half would remain while imperialism would be razed to the ground and the whole world would become socialist . " The Human Security Report 2005 documented a significant decline in the number and severity of armed conflicts since the end of the Cold War in the early 1990s . However , the evidence examined in the 2008 edition of the Center for International Development and Conflict Management ' s " Peace and Conflict " study indicated that the overall decline in conflicts had stalled . Throughout history . Nine largest ( by death toll ) . Three of the ten most costly wars , in terms of loss of life , have been waged in the last century . These are of course the two World Wars , then followed by the Second Sino - Japanese War ( which is sometimes considered part of World War II , or overlapping with that war ) . Most of the others involved China or neighboring peoples . The death toll of World War II , being 60 million plus , surpasses all other war - death - tolls . This may be due to significant recent advances in weapons technologies , as well as recent increases in the overall human population . Effects . On military personnel . Military personnel subject to combat in war often suffer mental and physical injuries , including depression , posttraumatic stress disorder , disease , injury , and death . During World War II , research conducted by US Army Brigadier General S . L . A . Marshall found that , on average , only 15 % to 20 % of American riflemen in WWII combat fired at the enemy . In Civil War Collector ’ s Encyclopedia , F . A . Lord notes that of the 27 , 574 discarded muskets found on the Gettysburg battlefield , nearly 90 % were loaded , with 12 , 000 loaded more than once and 6 , 000 loaded 3 to 10 times . These studies suggest that most military personnel resist firing their weapons in combat , that – as some theorists argue – human beings have an inherent resistance to killing their fellow human beings . Swank and Marchand ’ s WWII study found that after sixty days of continuous combat , 98 % of all surviving military personnel will become psychiatric casualties . Psychiatric casualties manifest themselves in fatigue cases , confusional states , conversion hysteria , anxiety , obsessional and compulsive states , and character disorders . Additionally , it has been estimated that anywhere from 18 % to 54 % of Vietnam war veterans suffered from Post Traumatic Stress Disorder . Based on 1860 census figures , 8 % of all white American males aged 13 to 43 died in the American Civil War , including about 6 % in the North and approximately 18 % in the South . The war remains the deadliest conflict in American history , resulting in the deaths of 620 , 000 military personnel . United States military casualties of war since 1775 have totaled over two million . Of the 60 million European military personnel who were mobilized in World War I , 8 million were killed , 7 million were permanently disabled , and 15 million were seriously injured . During Napoleon ' s retreat from Moscow , more French military personnel died of typhus than were killed by the Russians . Of the 450 , 000 soldiers who crossed the Neman on 25 June 1812 , less than 40 , 000 returned . More military personnel were killed from 1500 – 1914 by typhus than from military action . In addition , if it were not for modern medical advances there would be thousands more dead from disease and infection . For instance , during the Seven Years ' War , the Royal Navy reported that it conscripted 184 , 899 sailors , of whom 133 , 708 died of disease or were ' missing ' . It is estimated that between 1985 and 1994 , 378 , 000 people per year died due to war . On civilians . Most wars have resulted in significant loss of life , along with destruction of infrastructure and resources ( which may lead to famine , disease , and death in the civilian population ) . During the Thirty Years ' War in Europe , the population of the Holy Roman Empire was reduced by 15 to 40 percent . Civilians in war zones may also be subject to war atrocities such as genocide , while survivors may suffer the psychological aftereffects of witnessing the destruction of war . Most estimates of World War II casualties indicate that around 60 million people died , 40 million of which were civilians . Deaths in the Soviet Union were around 27 million . Since a high proportion of those killed were young men who had not yet fathered any children , population growth in the postwar Soviet Union was much lower than it otherwise would have been . On the economy . Once a war has ended , losing nations are sometimes required to pay war reparations to the victorious nations . In certain cases , land is ceded to the victorious nations . For example , the territory of Alsace - Lorraine has been traded between France and Germany on three different occasions . Typically speaking , war becomes very intertwined with the economy and many wars are partially or entirely based on economic reasons such as the American Civil War . Some economists believe war can stimulate a country ' s economy ( high government spending for World War II is often credited with bringing America out of the Great Depression by most Keynesian economists ) but in many cases , such as the wars of Louis XIV , the Franco - Prussian War , and World War I , warfare serves only to damage the economy of the countries involved . For example , Russia ' s involvement in World War I took such a toll on the Russian economy that it almost collapsed and greatly contributed to the start of the Russian Revolution of 1917 . World War II . One of the starkest illustrations of the effect of war upon economies is the Second World War . The Great Depression of the 1930s ended as nations increased their production of war materials to serve the war effort . The financial cost of World War II is estimated at about a trillion U . S . dollars worldwide , making it the most costly war in capital as well as lives . By the end of the war , the European economy had collapsed with 70 % of the industrial infrastructure destroyed . Property damage in the Soviet Union inflicted by the Axis invasion was estimated at a value of 679 billion rubles . The combined damage consisted of complete or partial destruction of 1 , 710 cities and towns , 70 , 000 villages / hamlets , 2 , 508 church buildings , 31 , 850 industrial establishments , of railroad , 4100 railroad stations , 40 , 000 hospitals , 84 , 000 schools , and 43 , 000 public libraries . On the arts . War leads to forced migration causing potentially large displacements of population . Among forced migrants there are usually relatively large shares of artists and other types of creative people , causing so the war effects to be particularly harmful for the country ’ s creative potential in the long - run . War is further argued to have a direct impact on artistic output , as it disrupts the production processes and distribution of artworks . Since creativity in the arts is often an expression of intense feeling , and as war affects the frame of mind of an artist , it has a negative effect on an artists ’ individual life - cycle output . It is not uncommon that during Wars , cultural institutions , such as libraries , are seen as “ targets in themselves ; their elimination was a way to denigrate and demoralize the enemy population . ” It is important to know about and understand the impact such destruction can have on a society because “ in an era in which competing ideologies fuel internal and international conflict , the destruction of libraries and other items of cultural significance is neither random nor irrelevant . Preserving the world ’ s repositories of knowledge is crucial to ensuring that the darkest moments of history do not endlessly repeat themselves . ” Factors ending a war . The political and economic circumstances , in the peace that follows war , usually depend on the facts on the ground . Where evenly matched adversaries decide that the conflict has resulted in a stalemate , they may cease hostilities to avoid further loss of life and property . They may decide to restore the antebellum territorial boundaries , redraw boundaries at the line of military control , or negotiate to keep or exchange captured territory . Negotiations between parties involved at the end of a war often result in a treaty , such as the Treaty of Versailles of 1919 , which ended the First World War . A warring party that surrenders or capitulates may have little negotiating power , with the victorious side either imposing a settlement or dictating most of the terms of any treaty . A common result is that conquered territory is brought under the dominion of the stronger military power . An unconditional surrender is made in the face of overwhelming military force as an attempt to prevent further harm to life and property . For example , the Empire of Japan gave an unconditional surrender to the Allies after the atomic bombings of Hiroshima and Nagasaki ( see Surrender of Japan ) and the preceding massive strategic bombardment of Japan . A settlement or surrender may also be obtained through deception or bluffing . Many other wars , however , have ended in complete destruction of the opposing territory , such as the Battle of Carthage of the Third Punic War between the Phoenician city of Carthage and Ancient Rome in 149 BC . In 146 BC the Romans burned the city , enslaved its citizens , and razed the buildings . Some wars or aggressive actions end when the military objective of the victorious side has been achieved . Others do not , especially in cases where the state structures do not exist , or have collapsed prior to the victory of the conqueror . In such cases , disorganised guerilla warfare may continue for a considerable period . In cases of complete surrender conquered territories may be brought under the permanent dominion of the victorious side . A raid for the purpose of looting may be completed with the successful capture of goods . In other cases an aggressor may decide to end hostilities to avoid continued losses and cease hostilities without obtaining the original objective , such as happened in the Iran – Iraq War . Some hostilities , such as insurgency or civil war , may persist for long periods of time with only a low level of military activity . In some cases there is no negotiation of any official treaty , but fighting may trail off and eventually stop after the political demands of the belligerent groups have been reconciled , a political settlement has been negotiated , the combatants are gradually killed or decide the conflict is futile , or the belligerents cease active military engagement but still threaten each other . A good example is the Chinese Civil War which was essentially over by 1950 , but in the second half of the 20th century the People ' s Republic of China began fighting to isolate the Republic of China ( commonly known as Taiwan ) diplomatically , and still sporadically threatens the island with an invasion . For this reason , some historians consider the war not ended but continuing . Ongoing conflicts . There are currently dozens of ongoing armed conflicts around the world , the deadliest of which is the Syrian Civil War . Efforts to limit or stop wars . See also : Aestheticization of violence Religious groups have long formally opposed or sought to limit war as in the Second Vatican Council document " Gaudiem et Spes " : " Any act of war aimed indiscriminately at the destruction of entire cities of extensive areas along with their population is a crime against God and man himself . It merits unequivocal and unhesitating condemnation . " Anti - war movements have existed for every major war in the 20th century , including , most prominently , World War I , World War II , and the Vietnam War . In the 21st century , worldwide anti - war movements occurred ever since the United States invaded Afghanistan and Iraq . In 2001 , the US government decided to invade Afghanistan to fight against international terrorism that caused the September 11 attacks . Protests opposing the War in Afghanistan occurred in cities in Europe , Asia , and all over the United States . Organizations like Stop the War Coalition , based in the United Kingdom , work on campaigning against the war . They raise awareness of the war , organize demonstrations , and lobby the governments . Significant worldwide opposition to the Iraq War also exists . Critics oppose the war based on the argument of violation of sovereignty , absence of the UN approval , and perceived illegitimacy . The Mexican Drug War , with estimated casualties of 40 , 000 since December 2006 , has been recently facing a fundamental opposition . In 2011 , the movement for peace and justice has started a popular middle - class movement against the war . It has won the recognition of President Calderon , who started the war , but has not ended it . Governments also use the method of disarmament to stop and prevent the cost of war . Theories for motivation . There is no scholarly agreement on which are the most common motivations for war . Motivations may be different for those ordering the war than for those undertaking the war . For example , in the Third Punic War , Rome ' s leaders may have wished to make war with Carthage for the purpose of eliminating a resurgent rival , while the individual soldiers may have been motivated by a wish to make money . Since many people are involved , a war may acquire a life of its own from the confluence of many different motivations . An interpretation of the ancient Jewish commentary ( BeReshit Rabbah ) on the fight between Cain and Abel in Genesis 4 ( Parashot BeReshit XXII : 7 ) states that there are three universal reasons for wars : A ) Economics , B ) Power , and C ) Religion . In " Why Nations Go to War " , by John G . Stoessinger , the author points out that both sides will claim that morality justifies their fight . He also states that the rationale for beginning a war depends on an overly optimistic assessment of the outcome of hostilities ( casualties and costs ) , and on misperceptions of the enemy ' s intentions . As the strategic and tactical aspects of warfare are always changing , theories and doctrines relating to warfare are often reformulated before , during , and after every major war . Carl von Clausewitz said , ' Every age had its own kind of war , its own limiting conditions , and its own peculiar preconceptions . ' The one constant factor is war ’ s employment of organized violence and the resultant destruction of property and / or lives that necessarily follows . Psychoanalytic psychology . Dutch psychoanalyst Joost Meerloo held that , " War is often . . . a mass discharge of accumulated internal rage ( where ) . . . the inner fears of mankind are discharged in mass destruction . " Thus war can sometimes be a means by which man ' s own frustration at his inability to master his own self is expressed and temporarily relieved via his unleashing of destructive behavior upon " others . " In this destructive scenario , these " others " are made to serve as the scapegoat of man ' s own unspoken and subconscious frustrations and fears . Other psychoanalysts such as E . F . M . Durban and John Bowlby have argued that human beings are inherently violent . This aggressiveness is fueled by displacement and projection where a person transfers his or her grievances into bias and hatred against other races , religions , nations or ideologies . By this theory , the nation state preserves order in the local society while creating an outlet for aggression through warfare . If war is innate to human nature , as is presupposed and predetermined by many psychological theories , then there is little hope of ever escaping it . The Italian psychoanalyst Franco Fornari , a follower of Melanie Klein , thought that war was the paranoid or projective “ elaboration ” of mourning . Fornari thought that war and violence develop out of our “ love need ” : our wish to preserve and defend the sacred object to which we are attached , namely our early mother and our fusion with her . For the adult , nations are the sacred objects that generate warfare . Fornari focused upon sacrifice as the essence of war : the astonishing willingness of human beings to die for their country , to give over their bodies to their nation . Despite Fornari ' s theory that man ' s altruistic desire for self - sacrifice for a noble cause is a contributing factor towards war , in history only a tiny fraction of wars have originated from a desire for war from the general populace . Far more often the general population has been reluctantly drawn into war by its rulers . One psychological theory that looks at the leaders is advanced by Maurice Walsh . He argues that the general populace is more neutral towards war and that wars only occur when leaders with a psychologically abnormal disregard for human life are placed into power . War is caused by leaders that seek war such as Napoleon and Hitler . Such leaders most often come to power in times of crisis when the populace opts for a decisive leader , who then leads the nation to war . Evolutionary . Several theories concern the evolutionary origins of warfare . There are two main schools : One sees organized warfare as emerging only in the Mesolithic , as a result of the emergence of complex social organization , higher population density and political organization and competition over resources . The other school tends to see human warfare simply as an extension of animal behavior , such as territoriality and sexual competition . This school argues that since organized warlike behavior patterns are also found in many other primate species such as chimpanzee s , as well as in many ant species , this suggests that between group conflict is a general feature of animal social behavior . Biologists studying primate behavior have added to the debate , documenting warlike activities among several primate species and seeing similarities to humans . Others argue that while war may be a natural phenomenon , the development of technology and complex social organization has accelerated the scale of warfare to exceptional levels among modern humans , starting at some point in the Mesolithic , and escalating with the development of weaponry and large - scale state formations . One line of evidence for violent conflict among the ancestors of humans is sexual dimorphism . In species that have high levels of male competition over females , males tend to be larger and stronger than females . Humans have considerable sexual dimorphism , although lower than our nearest primate relatives . The strength difference is greater for upper - body strength than for lower - body strength . Men are in general terms also larger , faster , and more aggressive . Their skeleton , especially in the vulnerable face , is more robust . This suggests that male competition has been an important factor in human evolution . Steven Pinker in his book " The Blank Slate " argues that raiding or warfare between groups of humans in the ancestral environment was often beneficial for the victors . This includes gaining control over scarce resources as well as the women of the defeated or raided group . Various features of modern warfare such as alliances between groups and preemptive war s were likely part of these conflicts . In order to have a credible deterrence against other groups ( as well as on an individual level ) , it was important to have a reputation for retaliation , causing humans to develop instincts for revenge as well as for protecting a group ' s ( or an individual ' s ) reputation ( " honor " ) . Pinker argues that the development of the state and the police have dramatically reduced the level of warfare and violence compared to the ancestral environment . Whenever the state breaks down , which can be very locally such as in poor areas of a city , humans again organize in groups for protection and aggression and concepts such as violent revenge and protecting honor again become extremely important . Ashley Montagu strongly denied universalistic instinctual arguments , arguing that social factors and childhood socialization are important in determining the nature and presence of warfare . Thus , he argues , while human aggression may be a universal occurrence , warfare is not , and would appear to have been a historical invention , associated with certain types of human societies . Bobbi S . Low also observed correlation between warfare and education , but noted that societies where warfare is commonplace encourage their children to be more aggressive , suggesting an opposite causality . Montagu ' s argument has been supported by ethnographic research conducted in societies where the concept of aggression seems to be entirely absent , e . g . , the Chewong of the Malay peninsula . Crofoot and Wrangham have instead argued that warfare , if defined as group interactions in which " coalitions attempt to aggressively dominate or kill members of other groups " , is a characteristic of most human societies . Those in which it has been lacking " tend to be societies that were politically dominated by their neighbors " . Economic . War can be seen as a growth of economic competition in a competitive international system . In this view wars begin as a pursuit of markets for natural resources and for wealth . While this theory has been applied to many conflicts , such counter arguments become less valid as the increasing mobility of capital and information level the distributions of wealth worldwide , or when considering that it is relative , not absolute , wealth differences that may fuel wars . There are those on the extreme right of the political spectrum who provide support , fascists in particular , by asserting a natural right of a strong nation to whatever the weak cannot hold by force . Some centrist , capitalist , world leaders , including Presidents of the United States and US Generals , expressed support for an economic view of war . Marxist . The Marxist theory of war is quasi - economic in that it states that all modern wars are caused by competition for resources and markets between great ( imperialist ) powers , claiming these wars are a natural result of the free market and class system . Part of the theory is that war will only disappear once a world revolution , over - throwing free markets and class systems , has occurred . Marxist philosopher Rosa Luxemburg theorized that imperialism was the result of capitalist countries needing new market s . Expansion of the means of production is only possible if there is a corresponding growth in consumer demand . Since the workers in a capitalist economy would be unable to fill the demand , producers must expand into non - capitalist markets to find consumers for their goods , hence driving imperialism . Demographic . Demographic theories can be grouped into two classes , Malthusian theories and youth bulge theories . Malthusian . Malthusian theories see expanding population and scarce resources as a source of violent conflict . Pope Urban II in 1095 , on the eve of the First Crusade , spoke : This is one of the earliest expressions of what has come to be called the Malthusian theory of war , in which wars are caused by expanding populations and limited resources . Thomas Malthus ( 1766 – 1834 ) wrote that populations always increase until they are limited by war , disease , or famine . This theory is thought by Malthusians to account for the relative decrease in wars during the past fifty years , especially in the developed world , where advances in agriculture have made it possible to support a much larger population than was formerly the case , and where birth control has dramatically slowed the increase in population . Youth bulge . Youth bulge theory differs significantly from Malthusian theories . Its adherents see a combination of large male youth cohorts – as graphically represented as a " youth bulge " in a population pyramid – with a lack of regular , peaceful employment opportunities as a risk pool for violence . While Malthusian theories focus on a disparity between a growing population and available natural resources , youth bulge theory focuses on a disparity between non - inheriting , ' excess ' young males and available social positions within the existing social system of division of labour . Contributors to the development of youth bulge theory include French sociologist Gaston Bouthoul , U . S . sociologist Jack A . Goldstone , U . S . political scientist Gary Fuller , and German sociologist Gunnar Heinsohn . Samuel Huntington has modified his Clash of Civilizations theory by using youth bulge theory as its foundation : Youth Bulge theories represent a relatively recent development but seem to have become more influential in guiding U . S . foreign policy and military strategy as both Goldstone and Fuller have acted as consultants to the U . S . Government . CIA Inspector General John L . Helgerson referred to youth bulge theory in his 2002 report " The National Security Implications of Global Demographic Change " . According to Heinsohn , who has proposed youth bulge theory in its most generalized form , a youth bulge occurs when 30 to 40 percent of the males of a nation belong to the " fighting age " cohorts from 15 to 29 years of age . It will follow periods with total fertility rate s as high as 4 – 8 children per woman with a 15 – 29 - year delay . A total fertility rate of 2 . 1 children born by a woman during her lifetime represents a situation in which the son will replace the father , and the daughter will replace the mother , accounting for a small proportion of deaths to factors such as illness and accidents . Thus , a total fertility rate of 2 . 1 represents replacement level , while anything below represents a sub - replacement fertility rate leading to population decline . Total fertility rates above 2 . 1 will lead to population growth and to a youth bulge . A total fertility rate of 4 – 8 children per mother implies 2 – 4 sons per mother . Consequently , one father has to leave not 1 , but 2 to 4 social positions ( jobs ) to give all his sons a perspective for life , which is usually hard to achieve . Since respectable positions cannot be increased at the same speed as food , textbooks and vaccines , many " angry young men " find themselves in a situation that tends to escalate their adolescent anger into violence : they are The combination of these stress factors according to Heinsohn usually heads for one of six different exits : Religions and ideologies are seen as secondary factors that are being used to legitimate violence , but will not lead to violence by themselves if no youth bulge is present . Consequently , youth bulge theorists see both past " Christianist " European colonialism and imperialism and today ' s " Islamist " civil unrest and terrorism as results of high birth rates producing youth bulges – with the Gaza Strip now being seen as another example of youth - bulge - driven violence , especially if compared to Lebanon which is geographically close , yet remarkably more peaceful . Among prominent historical events that have been linked to the existence of youth bulges is the role played by the historically large youth cohorts in the rebellion and revolution waves of early modern Europe , including the French Revolution of 1789 , and the importance of economic depression hitting the largest German youth cohorts ever in explaining the rise of Nazism in Germany in the 1930s . The 1994 Rwandan Genocide has also been analyzed as following a massive youth bulge . While the implications of population growth have been known since the completion of the National Security Study Memorandum 200 in 1974 , neither the U . S . nor the WHO have implemented the recommended measures to control population growth to avert the terrorist threat . Prominent demographer Stephen D . Mumford attributes this to the influence of the Catholic Church . Youth Bulge theory has been subjected to statistical analysis by the World Bank , Population Action International , and the Berlin Institute for Population and Development . Detailed demographic data for most countries is available at the international database of the United States Census Bureau . Statistic data about historical development of demographic and economic parameters over the last 200 years for each country can be visualized at Gapminder . Youth bulge theories have been criticized as leading to racial , gender and age " discrimination " . Rationalist . Rationalist theories of war assume that both sides to a potential war are rational , which is to say that each side wants to get the best possible outcome for itself for the least possible loss of life and property to its own side . Given this assumption , if both countries knew in advance how the war would turn out , it would be better for both of them to just accept the post - war outcome without having to actually pay the costs of fighting the war . This is based on the notion , generally agreed to by almost all scholars of war since Carl von Clausewitz , that wars are reciprocal , that all wars require both a decision to attack and also a decision to resist attack . Rationalist theory offers three reasons why some countries cannot find a bargain and instead resort to war : issue indivisibility , information asymmetry with incentive to deceive , and the inability to make credible commitments . Issue indivisibility occurs when the two parties cannot avoid war by bargaining because the thing over which they are fighting cannot be shared between them , only owned entirely by one side or the other . Religious issues , such as control over the Temple Mount in Jerusalem , are more likely to be indivisible than economic issues . A bigger branch of the theory , advanced by scholars of international relations such as Geoffrey Blainey , is that both sides decide to go to war and one side may have miscalculated . Looking at wars in history he argues , " war is usually the outcome of a diplomatic crisis which cannot be solved because both sides have conflicting estimates of their bargaining power . " Some scholars focus on information asymmetry with incentives to misrepresent . The two countries may not agree on who would win a war between them , or whether victory would be overwhelming or merely eked out , because each side has military secrets about its own capabilities . They will not avoid the bargaining failure by sharing their secrets , since they cannot trust each other not to lie and exaggerate their strength to extract more concessions . For example , Sweden made efforts to deceive Nazi Germany that it would resist an attack fiercely , partly by playing on the myth of Aryan superiority and by making sure that Hermann Göring only saw elite troops in action , often dressed up as regular soldiers , when he came to visit . The American decision to enter the Vietnam War was made with the full knowledge that the communist forces would resist them , but did not believe that the guerrillas had the capability to long oppose American forces . Thirdly , bargaining may fail due to the states ' inability to make credible commitments . In this scenario , the two countries might be able to come to a bargain that would avert war if they could stick to it , but the benefits of the bargain will make one side more powerful and lead it to demand even more in the future , so that the weaker side has an incentive to make a stand now . Rationalist explanations of war can be critiqued on a number of grounds . The assumptions of cost - benefit calculations become dubious in the most extreme genocidal cases of World War II , where the only bargain offered in some cases was infinitely bad . Rationalist theories typically assume that the state acts as a unitary individual , doing what is best for the state as a whole ; this is problematic when , for example , the country ' s leader is beholden to a very small number of people , as in a personalistic dictatorship . Rationalist theory also assumes that the actors are rational , able to accurately assess their likelihood of success or failure , but the proponents of the psychological theories above would disagree . Rationalist theories are usually explicated with game theory , for example , the Peace War Game , not a wargame as such , rather a simulation of economic decisions underlying war . Political science . The statistical analysis of war was pioneered by Lewis Fry Richardson following World War I . More recent databases of wars and armed conflict have been assembled by the Correlates of War Project , Peter Brecke and the Uppsala Conflict Data Program . The following subsections consider causes of war from system , societal , and individual levels of analysis . This kind of division was first proposed by Kenneth Waltz in " Man , the State , and War " ( 1959 ) and has been often used by political scientists since then . System - level theories . There are several different international relations theory schools . Supporters of realism in international relations argue that the motivation of states is the quest for security , and conflicts can arise from the inability to distinguish defense from offense , which is called the security dilemma . Within the realist and neorealist camps , two main sub - theories are While these two theories appear to contradict each other , they could both be correct depending on the system . For instance , balance - of - power theory might better describe Europe ' s history , while power - transition theory might better describe the world overall . Liberals in international relations cite other factors as relevant to conflicts , such as trade . If two countries have a profitable trading relationship , it ' s assumed that war would hurt both of them economically , making it less attractive . Realists respond that military force may sometimes be at least as effective as trade at achieving economic benefits , especially historically if not as much today . Also , trade can create dependence that allows for coercion , which can escalate conflict . Empirical data on the relationship of trade to peace are mixed , and moreover , some evidence suggests that countries at war don ' t necessarily trade less with each other . Individual - level theories . These theories suggest that differences in people ' s personalities , decision - making , emotions , belief systems , and biases are important in determining whether conflicts get out of hand . For instance , it has been proposed that conflict is modulated by bounded rationality and various cognitive biases , such as prospect theory . Other relevant factors can include ethic , moral , and religious differences , including declarations of independence by certain groups . War ethics . The seeming contradiction between warfare and morality has led to serious moral questions , which have been the subject of debate for thousands of years . The debate , generally speaking , has two main viewpoints : Pacifists , who believe that war is inherently immoral and therefore is never justified regardless of circumstances , and those who believe that war is sometimes necessary and can be moral . There are two different aspects to ethics in war , according to the most prominent and influential thought on justice and war : the Just War Theory . First is " jus ad bellum " ( literally translated as " right to war " ) , which dictates which unfriendly acts and circumstances justify a proper authority in declaring war on another nation . There are six main criteria for the declaration of a just war : first , any just war must be declared by a lawful authority ; second , it must be a just and righteous cause , with sufficient gravity to merit large - scale violence ; third , the just belligerent must have rightful intentions – namely , that they seek to advance good and curtail evil ; fourth , a just belligerent must have a reasonable chance of success ; fifth , the war must be a last resort ; and sixth , the ends being sought must be proportional to means being used . Once a just war has been declared , the second standard , or aspect , is put into effect . " Jus in bello " , which literally translates to " right in war " , are the ethical rules of conduct when conducting war . The two main principles in " jus in bello " are proportionality and discrimination . Proportionality regards how much force is necessary and morally appropriate to the ends being sought and the injustice suffered . The principle of discrimination determines who are the legitimate targets in a war , and specifically makes a separation between combatants , who it is permissible to kill , and non - combatants , who it is not . Failure to follow these rules can result in the loss of legitimacy for the just war belligerent , and so thereby forfeit the moral right and justice of their cause . The Just War standard is as old as Western Civilization itself , and still has significant impact on thinking about the morality of wars and violence today . Just War Theory was foundational in the creation of the United Nations and in International Law ' s regulations on legitimate war . These two positions generally cover the broad philosophical and ethical bents of mainstream society . However , there are several theories on and about war which are in the minority in culture , but which , because of the influence they have had in recent history , demand mention here . These strains of thought on human society and war can be broken up into two main camps : Marxist and Fascist , both of which view war as purely practical . Marxism , and other such historicist ideals , hold that history advances through a set of dialectics ( as stated by Heinrich Moritz Chalybäus : thesis , antithesis , synthesis ) . Marx , and his followers , in particular held that history advances through violence . Marxism – Leninism , in fact , held the belief that outright incitement to violence and war was necessary to topple capitalism and free the proletariat . In these theories , the question of ethics has no place , as the value of the war is entirely dependent on whether it advances the revolution or synthesis . Fascism , and the ideals it encompasses , such as Pragmatism , racism , and social Darwinism , hold that violence is good . Pragmatism holds that war and violence can be good if it serves the ends of the people , without regard for universal morality . Racism holds that violence is good so that a master race can be established , or to purge an inferior race from the earth , or both . Social Darwinism thinks that violence is sometimes necessary to weed the unfit from society so that civilization can flourish . These are broad archetypes for the general position that the ends justify the means . Social Darwinism as elaborated by the late U . S . sociologist and social evolutionist , William Sumner , states competition , conflict , inequality , and hierarchy is natural and good as it allows able - bodied and intelligent individuals and societies to lead and prosper . Lewis Coser , U . S . conflict theorist and sociologist , argued that conflict provides a function and a process whereby a succession of new equilibriums are created . Thus , the struggle of opposing forces , rather than being disruptive , may indeed be a means of balancing and maintaining a social structure or society .