War is a situation or a period of fighting between countries or groups of people. 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. Not every armed conflict is a war. A fight between individuals, between gangs, drug cartels, etc. is not considered a war. However, most wars are called armed conflicts. International humanitarian law is a set of rules that tries to limit the effects of wars. International Humanitarian Law recognizes two kinds of wars. These are:
- "International armed conflicts" between two or more states.
- "Non-international armed conflicts" as being between a government and a group that is not a government or one that is between two such groups.
Karl von Clausewitz wrote in his classic book, On War, that "war is a mere continuation of policy with other means.”[a] Clausewitz viewed war as a political instrument. His book about military philosophy remains the most influential work on the history and strategy of war. An earlier authority on war was Sun Tzu. In his book The Art of War, Sun Tzu saw war as a necessary evil. It was something people do.
Wars have been fought to control natural resources, for religious or cultural reasons and over political balances of power. 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. While a war can start for just about any reason, there is usually more than one cause.