An undeclared war is a conflict that is fought between two or more nations without a formal declaration of war being issued.
Since United Nations action in Korea, a number of democratic governments have pursued usually limited warfare by characterizing them as something else, such as a "military action" or "armed response." This was most notably used by the United States in its more than decade-long involvement in Vietnam. 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.
Not declaring war provides a way to circumvent constitutional safeguards against the executive declaring war, and also, in some cases, to avoid being bound by the established laws of war. Not using the word "war" is also seen as being more public relations-friendly. For these reasons, some nations have generally ceased to issue declarations of war, instead describing their actions by euphemisms such as "police action" or "authorized use of force."
Historical examples include the Quasi-War between the United States and France between 1798 and 1800, the Indian Wars of the American Old West, and the Anglo-Zulu War.
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.