A declaration of war is a formal act by which one state goes to war against another. The declaration is a performative speech act (or the 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. In addition to this, non-state or terrorist organizations may claim to or be described as "declaring war" when engaging in violent acts. These declarations may have no legal standing in themselves, but they may still act as a call to arms for supporters of these organizations.
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An alternative typology based upon the form of the declaration is formulated by Brien Hallett  according to 1) the degree to which the state and condition of war exists, 2) the degree of justification, 3) the degree of ceremony of the speech act, and 4) the degree of perfection of the speech act:
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 (UN) was established following that war in a renewed attempt to prevent international aggression through declarations of war.
In classical times, Thucydides condemned the Thebans, allies of Sparta, for launching a surprise attack without a declaration of war against Plataea, Athens' ally – an event that began the Peloponnesian War.
The utility of formal declarations of war has always been questioned, either as sentimental remnants of a long-gone age of chivalry or as imprudent warnings to the enemy. For example, writing in 1737, Cornelius van Bynkershoek judged that "nations and princes endowed with some pride are not generally willing to wage war without a previous declaration, for they wish by an open attack to render victory more honourable and glorious." Writing in 1880, William Edward Hall judged that "any sort of previous declaration therefore is an empty formality unless the enemy must be given time and opportunity to put himself in a state of defence, and it is needless to say that no one asserts such a quixotism to be obligatory."
The Hague Convention (III) of 1907 called "Convention Relative to the Opening of Hostilities" gives the international actions a country should perform when opening hostilities. The first two Articles say:
In 1989, Panama declared itself to be in a state of war with the United States. On 13 May 1998, at the outbreak of the Eritrean–Ethiopian War, Ethiopia, in what Eritrean radio described as a "total war" policy, mobilized its forces for a full assault against Eritrea. The Claims Commission found that this was in essence an affirmation of the existence of a state of war between belligerents, not a declaration of war, and that Ethiopia also notified the United Nations Security Council, as required under Article 51 of the UN Charter.
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, United Kingdom, or Russia when faced with a hostile invasion and/or occupation.
This 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.
|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||Egypt: 26 March 1979
Jordan: 26 October 1994
Syria: still at war
Iraq: still at war
Lebanon: still at war
|Ogaden War||13 July 1977||declaration of war||Somalia||Ethiopia||15 March 1978|
|Iran–Iraq War||22 September 1980||declaration of war||Iraq||Iran||20 July 1988|||
|Falklands War||11 May 1982||declaration of a war zone||Argentina||United Kingdom||20 June 1982|||
|United States invasion of Panama||15 December 1989||existence of a state of war||Panama||United States||31 January 1990|||
|Eritrean–Ethiopian War||14 May 1998||existence of a state of war||Ethiopia||Eritrea||12 December 2000|||
|Chadian Civil War (2005–10)||23 December 2005||existence of a state of war||Chad||Sudan||15 January 2010|||
|Djiboutian–Eritrean border conflict||13 June 2008||existence of a state of war||Djibouti||Eritrea||6 June 2010|||
|Russo-Georgian War||9 August 2008||existence of a state of war||Georgia||Russia||16 August 2008|||
|Heglig Crisis||11 April 2012||existence of a state of war||Sudan||South Sudan||26 May 2012|||
|Sinai insurgency||1 July 2015||existence of a state of war||Egypt||Islamic State||still at war|||
The United Nations Charter is the foundation of modern international law. The UN Charter is a treaty ratified by members of the UN, which are therefore legally bound by its terms. Article 2(4) of the UN Charter generally bans the use of force by states except when carefully circumscribed conditions are met, stating:
All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations.
This rule was "enshrined in the United Nations Charter in 1945 for a good reason: to prevent states from using force as they felt so inclined", said Louise Doswald-Beck, Secretary-General International Commission of Jurists.
Therefore, in the absence of an armed attack against a country or its allies, any legal use of force, or any legal threat of the use of force, has to be supported by a United Nations Security Council resolution authorizing member states to use force.
In an effort to force nations to resolve issues without warfare, framers of the United Nations Charter attempted to commit member nations to using warfare only under limited circumstances, particularly for defensive purposes.
The UN became a combatant itself after North Korea invaded South Korea on 25 June 1950 (see Korean War). The UN Security Council condemned the North Korean action by a 9–0 resolution (with the Soviet Union absent) and called upon its member nations to come to the aid of South Korea. The United States and 15 other nations formed a "UN force" to pursue this action. In a press conference on 29 June 1950, U.S. President Harry S. Truman characterized these hostilities as not being a "war" but a "police action".
The United Nations has issued Security Council Resolutions that declared some wars to be legal actions under international law, most notably Resolution 678, authorizing the 1991 Gulf War which was triggered by Iraq's invasion of Kuwait. UN Resolutions authorise the use of "force" or "all necessary means".
The examples and perspective in this article deal primarily with Western culture and do not represent a worldwide view of the subject. (January 2016) (Learn how and when to remove this template message)
Throughout the Commonwealth realms (the UK, Canada, et al.) the formal right to declare war rests with the monarch, currently Elizabeth II, as part of the royal prerogative (for example in the UK) or that realm's written constitution. In the United Kingdom parliamentary approval is often sought to deploy combat forces overseas, for example in the Iraq War and airstrikes on Daesh (ISIL), but this is not a legal requirement.
According to Article 35 of the French constitution, the French Parliament has the right to declare war. 
Article 115a says that unless attacked by an opposing military force, Germany must vote a two-thirds majority vote in the Bundestag if under the threat of war.
Article 28.3.1° of the Constitution of Ireland states that "war shall not be declared and the State shall not participate in any war save with the assent of Dáil Éireann." Ireland has taken a policy of non-alignment (what many confuse with neutrality see: Irish Neutrality) in military terms and is thus not a member of NATO.
According to the 11° article of the Italian Constitution, Italy rejects war as an instrument of aggression. Parliament has the power to declare war if is it necessary to create an order that ensures peace and justice among Nations; the most reliable authors exclude that among the circumstances in which it can be declared the state of war under Article 78 of the Constitution may be included also the state of internal civil war.
According to Article 9 of the Japanese Constitution war is unconstitutional.
According to Article 89 § VIII of the Mexican Constitution the President may declare war in the name of the United Mexican States after the correspondent law is enacted by the Congress of the Union.
According to the Spanish constitution of 1978, Art. 63, the King, with prior authorization by the Parliament, has the power to declare war and make peace.
According to 2010:1408 15 kap. 14 § entitled "Krigsförklaring" (declaration of war) the Swedish cabinet (regeringen) may not declare Sweden to be at war without the parliaments (riksdagen) consent unless Sweden is first attacked.
In the United States, Congress, which makes the rules for the military, has the power under the constitution to "declare war". However neither the U.S. Constitution nor any Act of Congress stipulate what format a declaration of war must take. War declarations have the force of law and are intended to be executed by the President as "commander in chief" of the armed forces. The last time Congress passed joint resolutions saying that a "state of war" existed was on June 5, 1942, when the U.S. declared war on Bulgaria, Hungary, and Romania. Since then, the US has used the term "authorization to use military force", as in the case against Iraq in 2003.
Sometimes decisions for military engagements were made by US presidents, without formal approval by Congress, based on UN Security Council resolutions that do not expressly declare the UN or its members to be at war. Part of the justification for the United States invasion of Panama was to capture Manuel Noriega (as a prisoner of war) because he was declared a criminal rather than a belligerent.
In response to the September 11 attacks, the United States Congress passed the joint resolution Authorization for Use of Military Force Against Terrorists on September 14, 2001, which authorized the US President to fight the War on Terror.
Developments in international law since 1945, notably the United Nations (UN) Charter, including its prohibition on the threat or use of force in international relations, may well have made the declaration of war redundant as a formal international legal instrument (unlawful recourse to force does not sit happily with an idea of legal equality).