A declaration of war is a formal act by which one state announces existing or impending war activity against another. The declaration is a performative speech act (or the public signing of a document) by an authorized party of a national government, in order to create a state of war between two or more states.

The legality of who is competent to declare war varies between nations and forms of government. In many nations, that power is given to the head of state or sovereign. In other cases, something short of a full declaration of war, such as a letter of marque or a covert operation, may authorise war-like acts by privateers or mercenaries. The official international protocol for declaring war was defined in the Hague Convention (III) of 1907 on the Opening of Hostilities.

Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations,^[1] though such declarations may have relevance within the domestic law of the belligerents or of neutral nations. The UN Security Council, under powers granted in articles 24 and 25, and Chapter VII of the Charter, may authorize collective action to maintain or enforce international peace and security. Article 51 of the United Nations Charter also states that: "Nothing in the present Charter shall impair the inherent right to individual or collective self-defence if an armed attack occurs against a state."^[2]

Declarations of war have been exceedingly rare since the end of World War II.^[3][4] Scholars have debated the causes of the decline, with some arguing that states are trying to evade the restrictions of international humanitarian law (which governs conduct in war)^[4] while others argue that war declarations have come to be perceived as markers of aggression and maximalist aims.^[3]

The practice of declaring war has a long history. The ancient Sumerian Epic of Gilgamesh gives an account of it,^[5] as does the Old Testament.^[6][7] The Roman Republic formalized the declaration of war by a special ceremony, the ritual of the Fetials, though the practice started to decline into the Imperial era.

However, the practice of declaring war was not always strictly followed. In his study Hostilities without Declaration of War (1883), the British scholar John Frederick Maurice showed that between 1700 and 1870 war was declared in only 10 cases, while in another 107 cases war was waged without such declaration (these figures include only wars waged in Europe and between European states and the United States, not including colonial wars in Africa and Asia).

In modern public international law, a declaration of war entails the recognition between countries of a state of hostilities between these countries, and such declaration has acted to regulate the conduct between the military engagements between the forces of the respective countries. The primary multilateral treaties governing such declarations are the Hague Conventions.

The League of Nations, formed in 1919 in the wake of the First World War, and the General Treaty for the Renunciation of War of 1928 signed in Paris, France, demonstrated that world powers were seriously seeking a means to prevent the carnage of another world war. Nevertheless, these powers were unable to stop the outbreak of the Second World War, so the United Nations was established following that war in a renewed attempt to prevent international aggression through declarations of war.

Declared wars since 1945

Declarations of war, while uncommon in the traditional sense, have mainly been limited to the conflict areas of the Western Asia and East Africa since 1945. Additionally, some small states have unilaterally declared war on major world powers such as the United States or Russia when faced with a hostile invasion and/or occupation. The following is a list of declarations of war (or the existence of war) by one sovereign state against another since the end of World War II in 1945. Only declarations that occurred in the context of a direct military conflict are included.

More information War(s), Date ...

War(s)	Date	Titled	Belligerents		Ended	References
			Declaring party	Opponent
Arab–Israeli War (1948–49) Suez Crisis (1956) Six-Day War (1967) War of Attrition (1967–70) Yom Kippur War (1973)	15 May 1948	Declaration of war	Egypt	Israel	26 March 1979	[11]
			Jordan		26 October 1994
			Syria, Iraq,  Lebanon		Still technically at war
Six-Day War (1967)	June 1967		Mauritania		1991	[12]
Ogaden War	13 July 1977		Somalia	Ethiopia	15 March 1978
Uganda–Tanzania War	2 November 1978		Tanzania	Uganda	3 June 1979	[13]
Iran–Iraq War	22 September 1980		Iraq	Iran	20 July 1988	[14]
United States invasion of Panama	15 December 1989	Existence of a state of war	Panama	United States	31 January 1990	[15]
Eritrean–Ethiopian War	14 May 1998		Ethiopia	Eritrea	12 December 2000	[16]
War on terror	23 February 1998	Declaration of war	al-Qaeda	Israel	Still at war	[17]
				United States
Chadian Civil War	23 December 2005	Existence of a state of war	Chad	Sudan	15 January 2010	[18]
Djiboutian–Eritrean border conflict	13 June 2008		Djibouti	Eritrea	6 June 2010	[19]
Russo-Georgian War	9 August 2008		Georgia	Russia	16 August 2008	[20]
Al-Qaeda insurgency in Yemen	14 January 2010		Yemen	al-Qaeda	Still at war	[21]
Heglig Crisis	11 April 2012		Sudan	South Sudan	26 May 2012	[22]
Sinai insurgency	1 July 2015		Egypt	Islamic State	25 January 2023	[23]
Anglophone Crisis	4 December 2017	Declaration of war	Cameroon	Ambazonia	Still at war	[24]
Second Nagorno-Karabakh War	27 September 2020	Existence of a state of war	Azerbaijan	Armenia	10 November 2020	[25]
Second Western Sahara War	14 November 2020	Declaration of war	SADR	Morocco	Still at war	[26]
Israel–Hamas war	7 October 2023		Israel	Hamas		[27]

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In Title II, Article 2 of the first Hague Convention of 1899, the signatory states agreed that at least one other nation be used to mediate disputes between states before engaging in hostilities:

In case of serious disagreement or conflict, before an appeal to arms, the signatory Powers agree to have recourse, as far as circumstances allow, to the good offices or mediation of one or more friendly Powers.^[33]

The Hague Convention (III) of 1907 called "Convention Relative to the Opening of Hostilities"^[34] gives the international actions a country should perform when opening hostilities. The first two Articles say:

Article 1

The Contracting Powers recognize that hostilities between themselves must not commence without previous and explicit warning, in the form either of a reasoned declaration of war or of an ultimatum with conditional declaration of war.^[35]

Article 2

The existence of a state of war must be notified to the neutral Powers without delay, and shall not take effect in regard to them until after the receipt of a notification, which may, however, be given by telegraph. Neutral Powers, nevertheless, cannot rely on the absence of notification if it is clearly established that they were in fact aware of the existence of a state of war.^[36]

The legality of who is competent to declare war varies between nations and forms of government. In many nations, that power is given to the head of state or sovereign. The official international protocol for declaring war was defined in the Hague Convention (III) of 1907 on the Opening of Hostilities.

Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations,^[1] though such declarations may have relevance within the domestic law of the belligerents or of neutral nations. The UN Security Council, under powers granted in articles 24 and 25, and Chapter VII of the Charter, may authorize collective action to maintain or enforce international peace and security. Article 51 of the United Nations Charter also states that: "Nothing in the present Charter shall impair the inherent right to individual or collective self-defence if an armed attack occurs against a state."^[2]

Declaring war is usually done through a process that involves prior approval before a formal announcement is made. This differs by country as some do not have a pre-approved process, and a given head of government can declare war with no pre-conditions.

• Ongoing wars (mostly undeclared)
• Frozen conflict
• List of wars extended by diplomatic irregularity
• Letter of protest
• State of emergency
• Undeclared war