Causes of World War II Out of all causes of World War II , the desire and ability of Adolf Hitler , in control of Nazi Germany , to dominate Europe and gain control especially of the agrarian resources to the east of Germany was the primary one . He was allied with the Empire of Japan , which desired to dominate Asia , including the much larger nation of China , as well as Italy ( which had ambitions to control parts of the Balkans ) and several smaller countries . Hitler had successfully taken control of Austria and Czechoslovakia by early 1939 , when Britain and France reversed their policy of appeasement and switched to a policy of deterrence , warning they would declare war if Germany attacked Poland . Hitler thought they were bluffing , and signed an agreement with the Soviet Union in late August that divided up Poland and the Baltic states . Germany invaded Poland on 1 September 1939 . Hitler ' s invasion of Poland drove Great Britain and France to declare war on Germany , and World War II had begun . Key events that led to the war included the 1939 invasion of Poland and the prior 1937 invasion of the Republic of China by Japan . The U . S . entered the war when it was attacked by Japan on 7 December 1941 . Ideologies , doctrines , and philosophies . The internationalist minded , radical Bolsheviks seized power in Russia in November 1917 and subsequently supported attempts to set up similar regimes elsewhere , with brief success in Hungary and Bavaria . This caused many central and western Europeans ( and Americans ) to fear that a violent Communist revolution would overwhelm their own countries . Beginning in 1919 the victorious " Entente " Powers established a " cordon sanitaire " of border states on Russia ' s western frontier in the hope of quarantining Communism in Russia . Both Italian and German fascism were in part a reaction to international communist socialist uprisings , in conjunction with nationalist fears of the Slavic empire . A further factor in Germany was the success of Freikorps ( voluntary paramilitary groups of discharged soldiers ) in crushing the Bolshevik Bavarian Soviet Republic in Munich in 1919 . Many of these veterans became early components of the Nazi s ' SA , which would be the party ' s troops in the street warfare with the Communist armed militia in the decade before 1933 . The street violence would help shift moderate opinion towards the need for Germany to find an anti - Communist strongman to restore stability to German life . Expansionism . Expansionism is the doctrine of expanding the territorial base ( or economic influence ) of a country , usually by means of military aggression . In Europe , Italy under Benito Mussolini sought to create a New Roman Empire based around the Mediterranean . It invaded Albania in early 1939 , at the start of the war , and later invaded Greece . Italy had also invaded Ethiopia as early as 1935 . This provoked angry words and an oil embargo from the League of Nations , which failed . Under the Nazi regime , Germany began its own program of expansion , seeking to restore the " rightful " boundaries of historic Germany , resulting in the remilitarization of the Rhineland . Also , of importance was the idea of a Greater Germany , supporters hoped to unite the German people under one nation state , which included all territories where Germans lived , disregarding the fact of them being minority in this territory . Germany ' s pre – World War II ambitions in both Austria and parts of Czechoslovakia mirror this goal . After the Treaty of Versailles , an Anschluss , or union , between Germany and a newly reformed German - Austria was prohibited by the Allies . Such a plan of unification , predating the creation of the German State of 1871 , had been discarded because of the Austro - Hungarian Empire ' s multiethnic composition as well as competition between Prussia and Austria for hegemony . At the end of World War I , the majority of Austrian Germans supported such a union . The Soviet Union had lost large parts of former Russian Empire territories to Poland , Finland , Estonia , Latvia , Lithuania and Romania in World War I and the Russian Civil War and was interested in regaining lost territories . Also during the Russo - Japanese war some territories had been lost to Japan . Hungary , an ally of Germany during World War I , had also been stripped of enormous territories after the partition of the Austria - Hungary empire and hoped to regain those lands by allying with Germany . Greater Hungary was a popular topic of discussion . Romania , while on the winning side in World War I , found itself on the losing side in early stages of World War II . As result of the Molotov - Ribbentrop Pact , Bessarabia and Northern Bukovina were ceded to the Soviet Union ; the Second Vienna Award resulted in the loss of Northern Transylvania to Hungary , and the Treaty of Craiova resulted in the return of Southern Dobruja to Bulgaria . Greater Romania was a concept that caused Romania to side more and more with Germany . Bulgaria , also an ally of Germany during World War I , had lost territories to Greece , Romania , and Yugoslavia in World War I and the Second Balkan War . Finland lost territory to the Soviet Union during the early stages of World War II in the lop - sided Winter War . When Germany attacked the Soviet Union in 1941 , Finland was drawn into what was called the Continuation War to regain what it had lost . In Asia , Japan harbored expansionist desires , fueled at least partially by the minimal gains the Japanese saw after World War I . Despite having taken a German colony in China and a few other Pacific islands , as well as swaths of Siberia and the Russian port of Vladivostok , Japan was forced to give up all but the few islands it had gained during World War I . Thailand had lost territories to France , the United Kingdom and Germany , at the end of 19th century and at the beginning of 20th century , and wanted to regain those areas . In many of these cases , the roots of the expansionism leading to World War II can be found in perceived national slights resulting from previous involvement in World War I , nationalistic goals of re - unification of former territories or dreams of an expanded empire . Fascism . Fascism is a philosophy of government that is marked by stringent social and economic control , a strong , centralized government usually headed by a dictator , and often has a policy of belligerent nationalism that gained power in many countries across Europe in the years leading up to World War II . In general , it believes that the government should control industry and people for the good of the country . In many ways , fascism viewed the army as a model that a whole society should emulate . Fascist countries were highly militaristic , and the need for individual heroism was an important part of fascist ideology . In his book " The Doctrine of Fascism " , Benito Mussolini declared that " fascism does not , generally speaking , believe in the possibility or utility of perpetual peace " . Fascists believed that war was generally a positive force for improvement and were therefore eager at the prospect of a new European war . Fascism ultimately proved to be one of the beliefs that was universal with many invading Axis countries Militarism . A highly militaristic and aggressive attitude prevailed among the leaders of Germany , Japan and Italy . Compounding this fact was the traditional militant attitude of the three had a similar track record that is often underestimated . For example , Germany introduced permanent conscription in 1935 , with a clear aim of rebuilding its army ( and defying the Treaty of Versailles ) . Racism . Twentieth - century events marked the culmination of a millennium - long process of intermingling between Germans and Slavs . Over the years , many Germans had settled to the east ( the Volga German s ) . Such migratory patterns created enclaves and blurred ethnic frontiers . By the 19th and 20th centuries , these migrations had acquired considerable political implications . The rise of the nation - state had given way to the politics of identity , including Pan - Germanism and Pan - Slavism . Furthermore , Social - Darwinist theories framed the coexistence as a " Teuton vs . Slav " struggle for domination , land and limited resources . Integrating these ideas into their own world - view , the Nazis believed that the Germans , the " Aryan race " , were the master race and that the Slavs were inferior . During World War II , Hitler used racism against " Non - Aryan " peoples . Interrelations and economics . Problems with the Treaty of Versailles . The Treaty of Versailles was neither lenient enough to appease Germany , nor harsh enough to prevent it from becoming the dominant continental power again . The treaty placed the blame , or " war guilt " on Germany and Austria - Hungary , and punished them for their " responsibility " rather than working out an agreement that would assure long - term peace . The treaty resulted in harsh monetary reparations , separated millions of ethnic Germans into neighboring countries , territorial dismemberment , caused mass ethnic resettlement and caused hyperinflation of the German currency ( see Inflation in the Weimar Republic ) . The Weimar Republic printed trillions of marks and borrowed heavily from the United States ( to later default ) to pay war reparations to Britain and France , who still carried war debt from World War I . The treaty created bitter resentment towards the victors of World War I , who had promised the people of Germany that U . S . President Woodrow Wilson ' s Fourteen Points would be a guideline for peace ; however , the US played a minor role in World War I and Wilson could not convince the Allies to agree to adopt his Fourteen Points . Many Germans felt that the German government had agreed to an armistice based on this understanding , while others felt that the German Revolution of 1918 – 1919 had been orchestrated by the " November criminals " who later assumed office in the new Weimar Republic . Contributing to this , following the Armistice of 1918 , Allied forces , including those of the American Army , occupied the Rhineland as far east as the river with some small bridgeheads on the east bank at places like Cologne . Under the terms of the Treaty of Versailles of 1919 the occupation was continued . The treaty specified three occupation Zones , which were due to be evacuated by Allied troops five , ten and finally 15 years after the formal ratification of the treaty , which took place in 1920 , thus the occupation was intended to last until 1935 . In fact , the last Allied troops left Germany five years prior to that date in 1930 in a good - will reaction to the Weimar Republic ' s policy of reconciliation in the era of Gustav Stresemann and the Locarno Pact . The German colonies were taken during the war , and Italy took the southern half of Tyrol after an armistice had been agreed upon . The war in the east ended with the collapse of Russian Empire , and German troops occupied ( with varying degree of control ) large parts of Eastern and Central Europe . After the destructive and indecisive battle of Jutland ( 1916 ) and the mutiny of its sailors in 1917 , the Kaiserliche Marine spent most of the war in port , only to be turned over to the allies and scuttled at surrender by its own officers . The lack of an obvious military defeat was one of the pillars that held together the " Dolchstosslegende " and gave the Nazis another tool at their disposal . An opposite view of the treaty held by some is that it did not go far enough in permanently neutering the capability of Germany to be a great power by dividing Germany into smaller , less powerful states . In effect , this would have undone Bismarck ' s work and would have accomplished what the French delegation at the Paris Peace Conference wanted . However , this could have had any number of unforeseeable consequences , especially amidst the rise of communism . Regardless , the Treaty of Versailles is generally agreed to have been a very poor treaty which helped the rise of the Nazi Party . French security demands . French security demands , such as reparations , coal payments , and a demilitarized Rhineland , took precedent at the Paris Peace Conference in 1919 and shaped the Treaty of Versailles by severely punishing Germany ; however , Germany found the treaty to be unjust which encouraged Hitler ’ s popularity . Paris Peace Conference ( 1919 ) . As World War I ended in 1918 , France , along with the other victor countries , were in a desperate situation regarding their economies , security , and morale . The Paris Peace Conference of 1919 was their chance to punish Germany for starting the war . The war “ must be someone ’ s fault – and that ’ s a very natural human reaction ” analyzed historian Margaret MacMillan . Germany was charged with the sole responsibility of starting World War I . The War Guilt Clause was the first step towards a satisfying revenge for the victor countries , namely France , against Germany . France understood that its position in 1918 was “ artificial and transitory ” . Thus , Clemenceau , the French leader at the time , worked to gain French security via the Treaty of Versailles . France ' s agenda during the Conference . The two main provisions of the French security agenda were reparations from Germany in the form of money and coal and a detached German Rhineland . The French government printed excess currency , which created inflation , to compensate for the lack of funds in addition to borrowing money from the United States . Reparations from Germany were necessary to stabilize the French economy . France also demanded that Germany give France their coal supply from the Ruhr to compensate for the destruction of French coalmines during the war . Because France feared for its safety as a country , the French demanded an amount of coal that was a “ technical impossibility ” for the Germans to pay back . France wanted the German Rhineland demilitarized because that would hinder a German attack . This gave France a physical security barrier between itself and Germany . The inordinate amount of reparations , coal payments , and the principle of a demilitarized Rhineland were viewed by the Germans to be insulting and unreasonable . Germany ' s reaction to Treaty of Versailles . “ No postwar German government believed it could accept such a burden on future generations and survive … ” . Paying reparations is a classic punishment of war but in this instance it was the “ extreme immoderation ” ( History ) that caused German resentment . Germany made its last World War I reparation payment on 3 October 2010 , ninety - two years after the end of World War I . Germany also fell behind in their coal payments . They fell behind because of a passive resistance movement against the French . In response , the French invaded the Ruhr , the region filled with German coal , and occupied it . At this point the majority of Germans were enraged with the French and placed the blame for their humiliation on the Weimar Republic . Adolf Hitler , a leader of the Nazi Party , attempted a coup d ’ état against the republic to establish a Greater German Reich known as the Beer Hall Putsch in 1923 . Although this failed , Hitler gained recognition as a national hero amongst the German population . The demilitarized Rhineland and additional cutbacks on military infuriated the Germans . Although it is logical that France would want the Rhineland to be a neutral zone , the fact that France had the power to make that desire happen merely added onto the resentment of the Germans against the French . In addition , the Treaty of Versailles dissolved the German general staff and possession of navy ships , aircraft , poison gas , tanks , and heavy artillery was made illegal . The humiliation of being bossed around by the victor countries , especially France , and being stripped of their prized military made the Germans resent the Weimar Republic and idolize anyone who stood up to it . Issues after the dissolution of Austria - Hungary . One major issue after the dissolution of Austria - Hungary was that the self - determination principle proposed by President Woodrow Wilson failed to achieve its goal . While some problems had been solved , a whole new set of issues emerged at the same time as a consequence of the treaties of Trianon and Saint Germain . Former lands of Austria - Hungary were divided up arbitrarily after the war in order to suit the ambitions of the victorious powers , and large groups of national minorities remained trapped in other countries . For example , a significant portion of Hungarians and Germans ended up under foreign rule . Hungary was held responsible for the war and stripped of two thirds of its territory and inhabitants ; while Austria , which had been an equal partner in the Austro - Hungarian government , received Burgenland ( formally part of Hungary ) , while losing the part of Tyrol that makes up Trentino - Alto Adige / Südtirol . German - speaking areas of Bohemia ( so - called Sudetenland ) remained part of it inside Czechoslovakia . In addition , Yugoslavia ( originally the Kingdom of Serbs , Croats , and Slovenes ) was home of five major , albeit related , ethnic groups ( Serbs , Croats , Macedons , Montenegrins , and the Slovenes ) , and was created after the war . Protectionist nationalistic policies of the successor states created high regional political tension and economic cooperation of the formerly united regions of Austria - Hungary was a thing of the past , which in the end , led to struggling development . As a result , irredentist and extremist movements gained strength and support from the population in this area . Competition for resources and markets . Other than a few coal and iron deposits , Japan lacks extensive natural resources . At the beginning of the twentieth century , Japan was a latecomer to the club of industrialized imperialist countries . By the time it had the ability to gain its own colonies , much of the Pacific and its resources had been carved up between the Western Great Powers : the British Empire included India , Singapore , Papua New Guinea ; the French empire included French Indochina ; and the Netherlands held the Dutch East Indies . In addition , the sphere of influence of the United States was expanding across the Pacific , annexing Hawaii , the Philippines , and providing crucial assistance to China . ( See Overseas expansion of the United States . ) At the start of the 20th century in the Russo - Japanese War , Japan had succeeded in pushing back the East Asian expansion of the Russian Empire in competition for Korea and Manchuria . The natural resources in Asia were shipped to fuel the industries of the colonial powers at low prices , often in closed economic systems such as the British Commonwealth , and were denied the Japanese industry . The markets for finished goods sent the colonies were also closed to the Japanese . According to Japanese diplomat Mamoru Shigemitsu , " The Japanese were completely shut out from the European colonies . In the Philippines , Indo - China , Borneo , Indonesia , Malaya , Burma , not only were Japanese activities forbidden , but even entry . Ordinary trade was hampered by unnatural discriminatory treatment . . . In a sense the Manchurian outbreak was the result of the international closed economies that followed on the first World War . There was a feeling at the back of it that it provided the only escape from economic strangulation . " The largest source both of raw material and consumers in Asia was China . Japan was determined to dominate this market , which the U . S . and other European powers had been dominating . On October 19 , 1939 , the American Ambassador to Japan , Joseph C . Grew , in a formal address to the America - Japan Society stated , " " What I shall say in Japan in the ensuing months ' comes straight from the horses mouth ' in that it will accurately represent and interpret some of the current thoughts of the American government and people with regard to Japan and the Far East . . . the American Government and people earnestly desire security , stability , and progress not only for themselves but for all other nations in every quarter of the world . But the new order in East Asia has appeared to include , among other things , depriving Americans of their long established rights in China , and to this the American people are opposed . . . American rights and interests in China are being impaired or destroyed by the policies and actions of the Japanese authorities in China . " In 1937 Japan invaded Manchuria and China proper . Under the guise of the Greater East Asia Co - Prosperity Sphere , with slogans as " Asia for the Asians ! " Japan sought to remove the Western powers ' influence in China and replace it with Japanese domination . The ongoing conflict in China led to a deepening conflict with the U . S . , where public opinion was alarmed by events such as the Nanking Massacre and growing Japanese power . Lengthy talks were held between the U . S . and Japan . When Japan moved into the southern part of French Indochina , President Roosevelt chose to freeze all Japanese assets in the U . S . The intended consequence of this was the halt of oil shipments from the U . S . to Japan , which had supplied 80 percent of Japanese oil imports . The Netherlands and UK followed suit . With oil reserves that would last only a year and a half during peace time ( much less during wartime ) , Japan had two choices : comply with the U . S . - led demand to pull out of China , or seize the oilfields in the East Indies from the Netherlands . The Japan government deemed it unacceptable to retreat from China . Hoping to knock out the United States for long enough to be able to achieve and consolidate their war - aims , the Japanese Navy attacked the U . S . Navy at Pearl Harbor on December 7 , 1941 . They mistakenly believed they would have a two - year window to consolidate their conquests before the United States could effectively respond , and the United States would seek a compromise peace long before the tide of war could potentially turn to the Allies ' superior production . Problems with the League of Nations . The League of Nations was an international organization founded after World War I to prevent future wars . The League ' s methods included disarmament ; preventing war through collective security ; settling disputes between countries through negotiation diplomacy ; and improving global welfare . The diplomatic philosophy behind the League represented a fundamental shift in thought from the preceding hundred years . The old philosophy , growing out of the Congress of Vienna ( 1815 ) , saw Europe as a shifting map of alliances among nation - states , creating a balance of power maintained by strong armies and secret agreements . Under the new philosophy , the League was a government of governments , with the role of settling disputes between individual nations in an open and legalist forum . The impetus for the founding of the League came from U . S . President Woodrow Wilson , though the United States never joined . This also lessened the power of the League — the addition of a burgeoning industrial and military world power would have added more force behind the League ' s demands and requests . The League lacked an armed force of its own and so depended on the members to enforce its resolutions , keep to economic sanctions which the League ordered , or provide an army , when needed , for the League to use . However , they were often very reluctant to do so . After numerous notable successes and some early failures in the 1920s , the League ultimately proved incapable of preventing aggression by the Axis Powers in the 1930s . The absence of the U . S . , the reliance upon unanimous decisions , the lack of an armed force , and the continued self - interest of its leading members meant that this failure was arguably inevitable . European Civil War . Some academics examine World War II as the final portion of a wider European Civil War that began with the Franco - Prussian War on July 19 , 1870 . The proposed period would include many ( but not all ) of the major European regime changes to occur during the period , including those during the Spanish Civil War and Russian Civil War . The Mason - Overy Debate : " The Flight into War " theory . In the late 1980s the British historian Richard Overy was involved in a historical dispute with Timothy Mason that mostly played out over the pages of the " Past and Present " journal over the reasons for the outbreak of World War II in 1939 . Mason had contended that a " flight into war " had been imposed on Adolf Hitler by a structural economic crisis , which confronted Hitler with the choice of making difficult economic decisions or aggression . Overy argued against Mason ' s thesis , maintaining that though Germany was faced with economic problems in 1939 , the extent of these problems cannot explain aggression against Poland and the reasons for the outbreak of war were due to the choices made by the Nazi leadership . Mason had argued that the German working - class was always opposed to the Nazi dictatorship ; that in the over - heated German economy of the late 1930s , German workers could force employers to grant higher wages by leaving for another firm that would grant the desired wage increases ; that this was a form of political resistance and this resistance forced Adolf Hitler to go to war in 1939 . Thus , the outbreak of the Second World War was caused by structural economic problems , a " flight into war " imposed by a domestic crisis . The key aspects of the crisis were according to Mason , a shaky economic recovery was threatened by a rearmament program that was overwhelming the economy and in which the Nazi regime ' s nationalist bluster limited its options . In this way , Mason articulated a " Primat der Innenpolitik " ( " primacy of domestic politics " ) view of World War II ’ s origins through the concept of social imperialism . Mason ' s " Primat der Innenpolitik " thesis was in marked contrast to the " Primat der Außenpolitik " ( " primacy of foreign politics ) usually used to explain World War II . In Mason ’ s opinion , German foreign policy was driven by domestic political considerations , and the launch of World War II in 1939 was best understood as a “ barbaric variant of social imperialism ” . Mason argued that “ Nazi Germany was always bent " at some time " upon a major war of expansion . ” However , Mason argued that the timing of a such a war was determined by domestic political pressures , especially as relating to a failing economy , and had nothing to do with what Hitler wanted . In Mason ' s view in the period between 1936 – 41 , it was the state of the German economy , and not Hitler ' s ' will ' or ' intentions ' that was the most important determinate on German decision - making on foreign policy . Mason argued that the Nazi leaders were deeply haunted by the November Revolution of 1918 , and was most unwilling to see any fall in working class living standards out of the fear that it might provoke another November Revolution . According to Mason , by 1939 , the “ overheating ” of the German economy caused by rearmament , the failure of various rearmament plans produced by the shortages of skilled workers , industrial unrest caused by the breakdown of German social policies , and the sharp drop in living standards for the German working class forced Hitler into going to war at a time and place not of his choosing . Mason contended that when faced with the deep socio - economic crisis the Nazi leadership had decided to embark upon a ruthless ' smash and grab ' foreign policy of seizing territory in Eastern Europe which could be pitilessly plundered to support living standards in Germany . Mason described German foreign policy as driven by an opportunistic ' next victim ' syndrome after the " Anschluss " , in which the “ promiscuity of aggressive intentions ” was nurtured by every successful foreign policy move . In Mason ’ s opinion , the decision to sign the German - Soviet Non - Aggression Pact with the Soviet Union and to attack Poland and the running of the risk of a war with Britain and France were the abandonment by Hitler of his foreign policy program outlined in " Mein Kampf " forced on him by his need to stop a collapsing German economy by seizing territory abroad to be plundered . For Overy , the problem with Mason ' s thesis was that it rested on the assumption that in a way not shown by records , information was passed on to Hitler about the " Reich " ' s economic problems . Overy argued that there was a difference between economic pressures induced by the problems of the Four Year Plan and economic motives to seize raw materials , industry and foreign reserves of neighboring states as a way of accelerating the Four Year Plan . Overy asserted that the repressive capacity of the German state as a way of dealing with domestic unhappiness was somewhat downplayed by Mason . Finally , Overy argued that there is considerable evidence that the German state felt they could master the economic problems of rearmament ; as one civil servant put it in January 1940 " we have already mastered so many difficulties in the past , that here too , if one or other raw material became extremely scarce , ways and means will always yet be found to get out of a fix " . Specific developments . Nazi dictatorship . Hitler and his Nazis took full control of Germany in 1933 - 34 , turning it into a dictatorship with a highly hostile outlook toward the Treaty of Versailles and Jews . It solved its unemployment crisis by heavy military spending . Hitler ' s diplomatic strategy was to make seemingly reasonable demands , threatening war if they were not met . When opponents tried to appease him , he accepted the gains that were offered , then went to the next target . That aggressive strategy worked as Germany pulled out of the League of Nations ( 1933 ) , rejected the Versailles Treaty and began to re - arm ( 1935 ) , won back the Saar ( 1935 ) , re - militarized the Rhineland ( 1936 ) , formed an alliance ( " axis " ) with Mussolini ' s Italy ( 1936 ) , sent massive military aid to Franco in the Spanish Civil War ( 1936 – 39 ) , seized Austria ( 1938 ) , took over Czechoslovakia after the British and French appeasement of the Munich Agreement of 1938 , formed a peace pact with Stalin ' s Russia in August 1939 , and finally invaded Poland in September 1939 . Re - militarization of the Rhineland . In violation of the Treaty of Versailles and the spirit of the Locarno Pact , Germany re - militarized the Rhineland on March 7 , 1936 . It moved German troops into the part of western Germany where , according to the Versailles Treaty , they were not allowed . France could not act because of political instability at the time . Britain thought the Versailles provision was unjust . Italian invasion of Ethiopia . Italian dictator Benito Mussolini attempted to expand the Italian Empire in Africa by invading the Ethiopian Empire ( also known as Abyssinia ) . To that time , Ethiopia had successfully resisted European colonization . With the pretext of the Walwal incident in late 1934 , Italy invaded on October 3 , 1935 without a formal declaration of war . The League of Nations declared Italy the aggressor and imposed sanctions which proved ineffective . On March 31 , 1936 , the Italians won the last major battle of the war , the Battle of Maychew . Emperor Haile Selassie fled into exile on May 2 . Italian forces took the capital , Addis Ababa , on May 5 . Italy annexed the Ethiopia on May 7 and merged Ethiopia , Eritrea , and Somaliland into a single colony known as Italian East Africa . On June 30 , 1936 , Emperor Haile Selassie gave a stirring speech before the League of Nations denouncing Italy ' s actions and criticizing the world community for standing by . He warned that " It is us today . It will be you tomorrow " . As a result of the League ' s condemnation of Italy , Mussolini declared the country ' s withdrawal from the organization . Spanish Civil War . Germany and Italy lent support to the Nationalists led by general Francisco Franco in Spain . The Soviet Union supported the existing government , the Spanish Republic . Both sides used this war as an opportunity to test improved weapons and tactics . The Bombing of Guernica was a horrific attack on civilians which foreshadowed events that would occur throughout Europe . The Nationalists won . Spain was neutral during World War II , but did send volunteer units to help the Germans against the USSR . Second Sino - Japanese War . The Second Sino - Japanese War began in 1937 when Japan attacked deep into China from its foothold in Manchukuo . The invasion was launched by the bombing of many cities such as Shanghai , Nanjing and Guangzhou . The latest , which began on 22 and 23 September 1937 , called forth widespread protests culminating in a resolution by the Far Eastern Advisory Committee of the League of Nations . The Imperial Japanese Army captured the Chinese capital city of Nanjing , and committed war crimes in the Nanjing massacre . Anschluss . The " Anschluss " was the 1938 annexation of Austria into Germany . Historically , the Pan - Germanism idea of creating a Greater Germany to include all ethnic Germans into one nation - state was popular for Germans in both Austria and Germany . It was at its peaking just after World War I when both new constitutions declared German Austria as a part of Germany . Such an action was expressly forbidden by the Treaty of Versailles , though . Nevertheless , Germany pressed for the Austrian Nazi Party ' s legality , played a critical role in the assassination of Austrian chancellor , Engelbert Dollfuss , and applied pressure for several Austrian Nazi Party members to be incorporated into offices within the Austrian administration . One of the Nazi party ' s National Socialist Program points was " " We demand the unification of all Germans in the Greater Germany on the basis of the people ' s right to self - determination . " " Adolf Hitler an Austrian German by birth stated in his book Mein Kampf " ( My Struggle ) " that he would create union between the two German states , his native birth country Austria and Germany . Following a Hitler speech at the Reichstag , Dollfuss ' successor , Kurt Schuschnigg , made it clear that he could be pushed " no further " . Amidst mounting pressures from Germany , he elected to hold a plebiscite , hoping to retain autonomy . However , just days prior to the balloting , a successful Austrian Nazi Party coup transferred power within the country . The takeover allowed German troops to enter Austria as " enforcers of the Anschluss " , since the Party quickly transferred power to Hitler . Consequently , no fighting occurred as most Austrians were enthusiastic , and Austria ceased to exist as an independent state . Britain , France and Fascist Italy , who all had vehemently opposed such a union , did nothing . Just as importantly , the quarreling amongst these powers doomed any continuation of a Stresa Front and , with no choice but to accept the unfavorable Anschluss , Italy had little reason for continued opposition to Germany , and was if anything drawn in closer to the Nazis . Munich Agreement . The so - called Sudetenland was a predominantly German - speaking region along the borders of Czechoslovakia with Germany . Its more than 3 million ethnic Germans comprised almost a quarter of the population of Czechoslovakia . After World War I , the Treaty of Versailles created two new states : Austria and Czechoslovakia . At the request of the Czechs the areas that would be known as the Sudetenland remained within Bohemia and were incorporated into the new Czechoslovak state against the wishes of much of the local population . The decision to disregard their right to self determination was based on French intent to weaken Austria and Germany . Parts of the so - called Sudetenland were highly industrialized , in fact one of the most industrialized regions in Czechoslovakia , and keeping it out of German influence would weaken the war potential of the former enemy . On October 31 , 1918 , future Czechoslovak President Tomáš G . Masaryk urged his ally Edvard Beneš , to " [ b ] e very careful – no weakness , but rather uncompromisingly demand full independence from the Habsburgs . . . Our Germans will be downcast when Germany surrenders ; insist on the historical - legal unity [ of Bohemia ] … It is more just to subjugate 3 million [ Germans ] than that 10 million [ Czechs and Slovaks ] be subjugated . " The Sudetenland contained most of the defensive system which ran across mountainous terrain and was larger than the Maginot line . The Sudetenland region also comprised about one third of Bohemia ( western Czechoslovakia ) in terms of territory , population , and economy . Czechoslovakia had a modern army of 38 divisions , backed by a well - noted armament industry ( Škoda ) as well as military alliances with France and Soviet Union . Hitler pressed for the Sudetenland ' s incorporation into the Reich , supporting German separatist groups within the Sudeten region . Alleged Czech brutality and persecution under Prague helped to stir up nationalist tendencies , as did the Nazi press . After the Anschluss , all German parties ( except German Social - Democratic party ) merged with the Sudeten German Party ( SdP ) . Paramilitary activity and extremist violence peaked during this period and the Czechoslovakian government declared martial law in parts of the Sudetenland to maintain order . This only complicated the situation , especially now that Slovakian nationalism was rising , out of suspicion towards Prague and Nazi encouragement . Citing the need to protect the Germans in Czechoslovakia , Germany requested the immediate annexation of the Sudetenland . In the Munich Agreement of September 30 , 1938 , British Prime Minister Neville Chamberlain and French leaders appeased Hitler . The conferring powers allowed Germany to move troops into the region and incorporate it into the Reich " for the sake of peace . " In exchange for this , Hitler gave his word that Germany would make no further territorial claims in Europe . Czechoslovakia , which had already mobilized over one million troops and was prepared to fight , was not allowed to participate in the conference . When the French and British negotiators informed the Czechoslovak representatives about the agreement , and that if Czechoslovakia would not accept it , France and Britain would consider Czechoslovakia to be responsible for war , President Edvard Beneš capitulated . Germany took the Sudetenland unopposed . German occupation and Slovak independence . In March 1939 , breaking the Munich Agreement , German troops invaded Prague , and with the Slovaks declaring independence , the country of Czechoslovakia disappeared . The entire ordeal was the last show of the French and British policy of appeasement . Italian invasion of Albania . After German occupation of Czechoslovakia , Italy saw itself becoming a second - rate member of the Axis . Rome delivered Tirana an ultimatum on March 25 , 1939 , demanding that it accede to Italy ' s occupation of Albania . King Zog refused to accept money in exchange for countenancing a full Italian takeover and colonization of Albania . On April 7 , 1939 , Mussolini ' s troops invaded Albania . Albania was occupied after a 3 days campaign with minimal resistance offered by the Albanian forces . Soviet – Japanese Border War . In 1939 , the Japanese attacked west from Manchuria into the Mongolian People ' s Republic , following the earlier Battle of Lake Khasan in 1938 . They were decisively beaten by Soviet units under General Georgy Zhukov . Following this battle , the Soviet Union and Japan were at peace until 1945 . Japan looked south to expand its empire , leading to conflict with the United States over the Philippines and control of shipping lanes to the Dutch East Indies . The Soviet Union focused on her western border , but leaving 1 million to 1 . 5 million troops to guard the frontier with Japan . Molotov - Ribbentrop Pact . Nominally , the Molotov - Ribbentrop Pact was a non - aggression treaty between Germany and the Soviet Union . It was signed in Moscow on August 23 , 1939 , by the Soviet foreign minister Vyacheslav Molotov and the German foreign minister Joachim von Ribbentrop . In 1939 , neither Germany nor the Soviet Union were ready to go to war with each other . The Soviet Union had lost territory to Poland in 1920 . Although officially labeled a " non - aggression treaty " , the pact included a secret protocol , in which the independent countries of Finland , Estonia , Latvia , Lithuania , Poland and Romania were divided into spheres of interest of the parties . The secret protocol explicitly assumed " " territorial and political rearrangements " " in the areas of these countries . Subsequently all the mentioned countries were invaded , occupied , or forced to cede part of their territory by either the Soviet Union , Germany , or both . Invasion of Poland . Between 1919 and 1939 Poland pursued a policy of balance between Soviet Union and Nazi Germany seeking non - aggression treaties with both In early 1939 Germany demanded that Poland join the Anti - Comintern Pact as a satellite state of Germany . Poland , fearing a loss of independence , refused , and Hitler told his generals to prepare an invasion for late summer while explaining to them that the real cause of war was not the city , but pursuit of Lebensraum . To deter Hitler , Britain and France announced that an invasion meant war , and tried to convince the Soviet Union to join in this deterrence . Moscow played along but found it could gain control of the Baltic states and parts of Poland by allying with Germany , which it did in August 1939 . London ' s deterrence had failed , but Hitler did not expect a wider war . Germany invaded Poland on September 1 , 1939 and rejected the British and French demands that it withdraw resulting in their declaration of war on September 3 , 1939 in accordance to the defense treaties they signed with Poland and publicly announced . Failed indirect contact . In 1939 - 40 during the " phoney war " the British Foreign Office sponsored several private contacts with Germany and Italy , looking for a way to open negotiations . Nothing developed and all contacts ended when the Germans launched their invasions of Denmark and Norway in April 1940 . One example was a 1940 trip to Italy made by British amateur diplomat James Lonsdale - Bryans . The trip , which was arranged with the support of Lord Halifax , was to meet with German ambassador Ulrich von Hassell . Lonsdale - Bryans talked of a deal whereby Germany would be given a free hand in Europe , while the British Empire would control the rest of the world . It is unclear to what extent this proposal enjoyed the official backing of the British Foreign Office . Invasion of the Soviet Union . Germany attacked the Soviet Union in June 1941 . Hitler believed that the Soviet Union could be defeated in a fast - paced and relentless assault that capitalized on the Soviet Union ' s ill - prepared state , and hoped that success there would bring Britain to the negotiation table , ending the war altogether . Hitler further wanted to preempt an attack by the Soviet Union , and in doing so catch the Soviets off - guard . Attack on Pearl Harbor . The Imperial Japanese Navy attacked Pearl Harbor on December 7 , 1941 , hoping to destroy the United States Pacific Fleet at anchor . Even though the Japanese knew that the U . S . had the potential to build more ships , they hoped that they would feed reinforcements in piecemeal and thus the Japanese Navy would be able to defeat them in detail . Japan ' s attack on the U . S . resulted in an immediate declaration of a state of war between the two nations . Although the Tripartite Pact with Germany and Italy did not mandate declaration of war if a signatory initiated an attack , Hitler chose to declare that the Pact required that Germany follow Japan ' s declaration of war . Germany and Italy did not have to declare war on the USA in order to fulfill their treaty obligations , but eagerly did so . Within days , Germany and Italy declared war on the United States , effectively ending isolationist sentiment in the U . S . which had so far prevented it from entering the war .