Undeclared war

An undeclared war is a military conflict between two or more nations without either side issuing a formal declaration of war. The term is sometimes used to include any disagreement or conflict fought about without an official declaration. Since the United Nations police action in Korea, some governments have pursued disciplinary actions and limited warfare by characterizing them as something else such as a military action or armed response.

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 August 2013, 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 retaliatory invasion against South Korea. The conflict resulted in over 142,000 American casualties (about 40,000 deaths and over 100,000 injuries).

Russia
No formal declaration of war has been issued in the ongoing Russo-Ukrainian War. When Russia's president Vladimir Putin announced the Russian invasion of Ukraine in 2022, he announced the start of a "Special military operation", side-stepping a formal declaration of war.

United Kingdom
The Falklands War between Argentina and the United Kingdom (April - June 1982) was never formally declared as a war.