Declaration of war

A Declaration of War is a formal declaration issued by a national government indicating that a state of war exists between that nation, and one or more others.

Declarations of war were common in the late 19th and early 20th Century, but have largely been phased out as a diplomatic tool since the end of World War II. Among other reasons, this is because the legal concept of a "state of war" brings with it many logistical complications involving the established laws of war and other complex political issues. However, this isn't to say that the absence of a declaration of war means that a military conflict will be chaotic and unlawful; in many cases the rules of war are now well enough accepted to make formal declarations unessisary.

Instead of formal war declarations, many countries now just issue "authorizations of force" to the military, and diplomatic warnings to their enemy nations.