War is a situation or a period of fighting between countries or groups of people.[1] A war generally involves the use of weapons, a military organization and soldiers. War is a situation in which a nation enforces its rights by using force.[2] Not every armed conflict is a war. A fight between individuals, between gangs, drug cartels, etc. is not considered a war.[3] However, most wars are called armed conflicts. International humanitarian law is a set of rules that tries to limit the effects of wars.[4] International Humanitarian Law recognizes two kinds of wars. These are:

Karl von Clausewitz wrote in his classic book, On War, that "war is a mere continuation of policy with other means.”[a][6] Clausewitz viewed war as a political instrument.[8] His book about military philosophy remains the most influential work on the history and strategy of war.[9] An earlier authority on war was Sun Tzu.[8] In his book The Art of War, Sun Tzu saw war as a necessary evil.[8] It was something people do.[8]

Wars have been fought to control natural resources, for religious or cultural reasons and over political balances of power.[10] They have been fought over legitimacy (correctness) of particular laws. They have been fought to settle arguments about land or money, and many other issues. The reasons behind any war are often very complex.[11] While a war can start for just about any reason, there is usually more than one cause.[12] War is also a terrible coincidence to be through

From the earliest times, individual states or political factions have used war to gain sovereignty over regions.[13] In one of the earliest civilizations in history, Mesopotamia, they were in a near constant state of war.[13] Ancient Egypt during its Early Dynastic Period came about by war when Lower and Upper Egypt were joined together as one country, about 3100 BC.[13] The Zhou Dynasty ruled Ancient China came to power in 1046 through war.[14] Scipio Africanus (236-183 BCE) defeated Carthage leading Ancient Rome to begin a conquest of the known world.[13] Philip II of Macedon (382-336 BCE) united a group of city-states to become Ancient Greece.[13]

Sometimes, people don't see a difference between fighting between countries or people, and the formal declaration of a state of war. Those who do see this difference usually only use the word "war" for the fighting where the countries' governments have officially declared war on each other. Smaller armed conflicts are often called riots, rebellions, coups, etc.

One country may send forces to another country for a variety of reasons. Sometimes it is to help keep order or prevent killings of innocents or other crimes against humanity. It may be to protect a friendly government against an uprising. Here it may be called a police action or humanitarian intervention instead of a war. Some people think it's still a war.

Another kind of war existed from 1947 until 1991 called the Cold War.[15] This started when diplomatic relations between the United States and the Soviet Union broke down.[15] Both countries had nuclear weapons and both stood ready to use them against the other.[15] But there was no actual war between the two. It ended with the fall of the Soviet Union in 1991.[15] The cold war was also called a containment where the United States tried to prevent the spread of communism to other countries.[16] During the cold war, the major powers did not fight themselves, but often backed third parties in what was called a proxy war.[17] The Vietnam War is often given as an example of a proxy war.[17] But proxy wars happened long before the cold war and are still happening.[17]

A war between peoples and groups in the same country is known as a civil war.[18] It is generally agreed there are two things that make a war a civil war.[18] It must be a struggle between groups in the same country or state over political control or to force a major change in the government's policy.[18] The second criterion is that more than 1000 people have to have been killed, with a minimum of 100 from each side.[18] The American Civil War is an example of a civil war. While the figures are mere estimates, the total casualties are thought to be about 750,000.[19]

Only in the last 150 years or so, have states agreed on international laws to limit warfare.[20] This has been mainly for humanitarian reasons.[20] The Geneva conventions and the Hague Conventions are two examples of agreements that establish laws governing wars.[21] Collectively, these are usually called International humanitarian law (IHL).[21] Because these are established laws, they restrict those engaged in armed conflicts to follow the IHL.[20] Also, a country must not only respect the law but they also need to make sure other countries respect it as well.[20] They cannot turn a blind eye (meaning pretend they do not see a thing) to countries who are not following IHC.[20] The first of these was the Geneva Convention in 1864.[22] It became international law with the signatures of 100 countries.[23]

The statistical analysis of war was started by Lewis Fry Richardson following World War I. More recent databases of wars have been assembled by the Correlates of War Project and Peter Brecke.