Undeclared war

Under customary international law, it is not necessary to declare war—simply beginning hostilities is sufficient to make belligerent intentions clear.

United States

There is no specific format required under United States law for the way an official war declaration will be structured or delivered. The United States Constitution states: "The Congress shall have Power […] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water".

As of September 2024, the United States Congress has formally declared war 11 times, and has not done so since 1942; 6 of these were WWII declarations. The United States did not declare war during its involvement in Vietnam, although the Gulf of Tonkin Resolution authorized the escalation and use of military force in the Vietnam War without a formal declaration of war. On at least 125 occasions a US president has employed military forces without authorization from Congress. One of the most significant of these occasions was the Korean War, where the United States led a peacekeeping United Nations force to stop North Korea's invasion against South Korea. The conflict resulted in over 142,000 American casualties (about 40,000 deaths and over 100,000 injuries).