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 followed the example set by the United Kingdom during the so-called Malayan Emergency, a number of democratic governments have pursued disciplinary actions and limited warfare by characterizing them as something else such as a military action or armed response.
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 only formally declared war 13 times. The United States never officially declared war during its more than decade-long 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, a conflict that resulted in over 142,000 American casualties.