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, 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." The United States has not formally declared war since World War II. 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. 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, since they could no longer be deemed internal conflicts. Conducting military actions without declaring war may offer a way to circumvent constitutional requirements for declarations of war and avoid being bound by the established laws of war.

Examples
Historical examples include the Quasi-War between the United States and France between 1798 and 1800, the Indian Wars of the American Old West, Anglo-Zulu War and the 1982 Falklands War between Great Britain and Argentina.

The Quasi-War was a conflict between the France and the United States after the French Revolution. Once the new French Republic leadership was in place, the Americans stopped repaying their debt to France with the justification that it was owed to the monarchy of France and not the Republic. In response, France attacked U.S. trade ships, leading to the Quasi-War, which began on July 7, 1798

United States
The United States Constitution says "Congress shall have the power to ... declare War, ...", but does not define the form such declarations will take. Therefore, some have argued that congressionally passed authorizations to use military force are in fact declarations of war. This was supported by the War Powers Resolution of 1973 (50 U.S.C.A. §§ 1541 et seq.) which requires the president to consult with Congress before introducing armed forces into hostilities.