Undeclared War is a 1990 Hong Kong action film directed by Ringo Lam and starring Danny Lee, Olivia Hussey, Peter Lapis, Rosamund Kwan and Vernon G. Wells.

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.