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 US or International Law for the way an official war declaration will be structured or delivered. Instead the US Constitution says only "The Congress shall have Power To ... To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water." Nations such as France, which had extensive colonies in which its military provided order, continued to intervene in their former colonies' affairs as police actions.