Declaration of war A declaration of war is a formal act by which one nation 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 , 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 privateer s 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 use of force in international conflicts , have made declarations of war largely obsolete in international relations . In addition to this , non - state or terrorist organisations may claim to or be described as " declaring war " when engaging in violent acts . These declarations may have no legal standing in themselves , but may still act as a call to arms for supporters of these organisations . Definitions . Theoretical perspectives . A definition of the three ways of thinking about a declaration of war was developed by Saikrishna Prakash . He argues that a declaration of war can be seen from three perspectives : Types of declarations . 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 : History . The practice of declaring war has a long history . The ancient Sumerian Epic of Gilgamesh gives an account of it , as does the Old Testament . 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 . Formal Declarations of War during World War II . 3 September 1939 – United Kingdom , India , France , Australia and New Zealand declared war on Germany . < br > 6 September 1939 – South Africa declared war on Germany . < br > 10 September 1939 – Canada declared war on Germany . < br > 24 April 1940 - Germany declared war on Norway < br > 10 May 1940 – Netherlands declared war on Germany . < br > 10 June 1940 – Italy declared war on UK and France . < br > 23 November 1940 – Belgian government in exile declared war on Italy . < br > 25 June 1941 – Finland declared war on the Soviet Union . < br > 27 June 1941 – Hungary declared war on the Soviet Union . < br > 7 December 1941 – Empire of Japan declared war on the United States of America , United Kingdom , Australia , New Zealand , and South Africa . < br > 7 December 1941 – United Kingdom , Canada , Australia and New Zealand declared war on Finland , Romania and Hungary . < br > 7 December 1941 – Panama declared war on Japan . < br > 8 December 1941 – The United States of America , United Kingdom , Canada , Australia , Costa Rica , The Dominican Republic , El Salvador , Haiti , Honduras , The Netherlands , New Zealand and Nicaragua declare war on Japan . < br > 9 December 1941 – Republic of China declared war on Japan , Italy and Germany . < br > 11 December 1941 – Germany and Italy declared war on the United States of America . < br > 11 December 1941 – The United States of America declared war on Germany and Italy . < br > 11 December 1941 – Polish government in exile declared war on Japan . < br > 12 December 1941 – Bulgaria , Hungary , Romania and Slovakia declare war on the United States of America and United Kingdom . < br > 13 December 1941 – United Kingdom , New Zealand and the Union of South Africa declare war on Bulgaria . < br > 14 December 1941 – Croatia declared war on the United States of America and United Kingdom . < br > 16 December 1941 – Czechoslovak government in exile declares war on all countries at war with the United States of America , United Kingdom and the Soviet Union . < br > 17 December 1941 – Albania ( under Italian occupation ) declared war on the United States of America . < br > 19 December 1941 – Nicaragua declared war on Bulgaria , Hungary and Romania . < br > 20 December 1941 – Belgium declared war on Japan . < br > 6 January 1942 – Australia declared war on Bulgaria . < br > 25 January 1942 – United Kingdom , New Zealand and the Union of South Africa declare war on Thailand . < br > 22 May 1942 – Mexico declared war on Germany , Italy and Japan . < br > 22 August 1942 – Brazil declared war on Germany and Italy . < br > 2 April 1943 – Bolivia declared war on all Axis members . < br > 9 September 1943 – Iran declared war on Germany . < br > 13 October 1943 – Italy ( after switching sides ) declared war on Germany . < br > 24 October 1943 - Provisional Government of Free India declared war on Britain , and the United States of America . < br > 25 August 1944 – Romania ( after switching sides ) declared war on Germany . < br > 21 September 1944 – San Marino declared war on Germany . < br > 22 September 1944 - The government of the Second Philippine Republic declares war on the United States and Great Britain . The declaration took effect the following day . < br > 7 February 1945 – Argentina and Paraguay declared war on Germany and Japan . < br > 12 February 1945 – Peru in state of belligerency with Germany and Japan . < br > 15 February 1945 – Venezuela and Uruguay declare war on Germany and Japan . < br > 23 February 1945 – Turkey declared war on Germany and Japan . < br > 24 February 1945 – Egypt declared war on Germany and Japan . < br > 26 February 1945 – Syria declared war on Germany and Japan . < br > 1 March 1945 – Saudi Arabia declared war on Japan . < br > 3 March 1945 – Finland ( after switching sides ) declared war on Germany . < br > 11 April 1945 – Chile declared war on Japan . < br > 6 July 1945 - Norway declared war on Japan < br > After World War II . 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 Ethiopian - Eritrean 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 . In December 2005 the government of Chad declared that a state of war existed with Sudan , after Sudan hosted Chadian rebel groups that were behind fatal cross border raids . In 2008 , after armed clashes broke out during the Djiboutian – Eritrean border conflict , Djibouti ' s President Guelleh , when asked if his country was at war with Eritrea , replied with " absolutely " . On 11 April 2012 , Sudan declared war on South Sudan after weeks of border clashes . 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 , 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 . United Nations and war . 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 war 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 " . Denigration of formal 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 touched off 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 . " United States . 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 the law 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 5 June 1942 , when the U . S . declared war on Bulgaria , Hungary , and Romania . Since then , the U . S . 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 attacks on 11 September 2001 , the United States Congress passed the joint resolution Authorization for Use of Military Force Against Terrorists on 14 September 2001 , which authorized the US President to fight the War on Terror . Agreed Procedure for the Opening of Hostilities . 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 :