Carl von Clausewitz wrote in his classic text, On War: "Der Krieg ist eine bloße Fortsetzung der Politik mit anderen Mitteln" ("War is merely a continuation of politics by other means") and "War is thus an act of force to compel our enemy to do our will."
Wars have been fought to control natural resources, for religious or cultural reasons, over political balances of power, legitimacy of particular laws, to settle economic and territorial disputes, and many other issues. The roots of any war are very complex - there is usually more than one issue involved.
Sometimes a distinction is made between a conflict and the formal declaration of a state of war. Those who make this distinction often restrict the term "war" to those conflicts where the countries have formally declared such a state. Smaller armed conflicts are often called riots, rebellions, coups, etc.
When one country sends armed forces to another allegedly to restore order or prevent genocide or other crimes against humanity, or to support a legally recognized government against insurgency, that country sometimes refers to it as a police action[?]. This usage is not always recognized as valid, however, particularly by those who do not accept the connotations of the term.
A war where the forces in conflict belong to the same country or empire or other political entity is known as a civil war.
War is contrasted with peace, which is usually defined as the absence of war.
A number of treaties regulate warfare, collectively referred to as the Laws of war. The most pervasive of those are the Geneva conventions, the earliest of which began to take effect in the mid 1800s.
Treaty signing has since been a part of international diplomacy, and too many treaties to mention in this scant article have been signed. A couple of examples are: Resolutions of the Geneva International Conference, Geneva, 26-29 October 1863 and Geneva Convention relative to the Treatment of Prisoners of War, 75 U.N.T.S. 135, entered into force Oct. 21, 1950.
The statistical analysis of war was pioneered 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 .