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.
While some have claimed the United States has not formally declared war since World War II and/or most notably, the United States never officially declared war during its more than decade-long involvement in Vietnam; the Gulf of Tonkin Resolution authorized the escalation of the Vietnam War without a declaration of war.
There is no specific format required under United States or international law for the way an official war declaration will be structured or delivered. Instead, the United States Constitution says: "The Congress shall have Power [...] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water".