Declaration of war

A formal announcement by a state that it is entering a state of war with another state. It can mark a legal and political transition, though armed conflict may begin without one.

War: Organized armed conflict between political communities, usually involving states or groups seeking political aims. A declaration announces entry into this broader condition of organized armed conflict.
Just war theory: A tradition of ethical reasoning about when war may be started and how it may be fought. Its criteria shaped arguments about whether rulers could justly announce and wage war.
Undeclared war: Armed conflict conducted without a formal declaration of war by the states involved. It shows that states can fight while avoiding the formal announcement this concept denotes.
War powers: The constitutional and statutory authorities governing a government's decision to use military force. A declaration may trigger or clarify domestic powers assigned to the executive and legislature.
Ultimatum: A final demand backed by a stated threat, often requiring action by a deadline. States may issue an ultimatum before declaring war, making its demands and deadline part of the crisis.
Roman fetial law: Ritual and legal procedures through which ancient Rome formally demanded redress and declared public war. Roman rites are an early model for treating war declarations as formal acts by political authorities.
Use of force: The employment of armed force by states, governed in international law by rules including the UN Charter. International law regulates force whether or not a state has declared war.
Trading with the Enemy Act: A United States federal law authorizing restrictions on trade and financial dealings with an enemy during wartime. A formal state of war can activate legal restrictions on commerce with the opposing country.
Casus belli: An act or event presented as justification for starting a war. Governments commonly cite a casus belli to explain and defend a declaration.
Grotius: A Dutch jurist whose writings helped shape modern international law, especially the law of war and peace. His work helped systematize the legal framework in which declarations and hostilities were understood.

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Ottoman entry into World War IRelated: The Ottoman declaration against the Entente followed the Black Sea hostilities.