This is a timeline of declarations of war during World War II.

A declaration of war is a formal act by which one nation goes to war against another. The declaration is usually the act of delivering a performative speech or the presentation of a signed document by an authorized party of a national government in order to create a state of war between two or more sovereign states. The official international protocol for declaring war was defined in The Hague Peace Conference of 1907 (or Hague II).^[1] For the diplomatic maneuvering behind these events, which led to hostilities between nations during World War II, see Diplomatic history of World War II.

Below is a table showing the outbreak of wars between nations which occurred during World War II. Indicated are the dates (during the immediate build-up to, or during the course of, World War II), from which a de facto state of war existed between nations. The table shows both the "Initiator Nation(s)" and the nation at which the aggression was aimed, or "Targeted Nation(s)". Events listed include those in which there were simple diplomatic breaking of relations that did not involve any physical attack, as well as those involving overt declarations or acts of aggression. In rare cases, war between two nations occurred twice, with an intermittent period of peace. The list here does not include peace treaties or periods of any armistice.

Outbreaks of war between nations during World War II

Date	Initiator nation(s)	Targeted nation(s)	Type	Notes/comments	Document/event
1939-09-01	Germany	Poland	A	German attack began at 4:44 a.m., Berlin and Warsaw time.[2][better source needed][3][4] Germany claimed that the attack was defensive, citing the Gleiwitz incident, which was actually a false flag attack.[5]	Invasion
1939-09-01	Slovakia	Poland	A		Invasion
1939-09-03	United Kingdom  Australia  New Zealand  India Tonga Transjordan	Germany	U[3][6]	At 11:15 a.m. London time,[7] British PM, Neville Chamberlain publicly delivered his Ultimatum Speech.[a][6][8] As the Statute of Westminster 1931 was not yet ratified by the parliaments of Australia and New Zealand, the British declaration of war on Germany also applied to those dominions. Tonga was not a British dominion, instead, as part of the British Commonwealth (a protected state of the UK and the British Empire) declared war separately, alongside Britain (since it administers their foreign affairs).[9]	United Kingdom declaration
1939-09-03	France Morocco Tunisia	Germany	U	The French ultimatum to Germany expired a few hours after the British ultimatum, at 17:00.	French declaration
1939-09-04	Nepal	Germany	W		Declaration
1939-09-06	South Africa	Germany	W[3][6]		Declaration
1939-09-10	Bahrain  Canada Oman	Germany	W[3][6]	Canada's declaration several days after that of the United Kingdom is seen as a watershed moment in Canadian home rule and sovereignty. As for Bahrain, given its status as a protected state of the United Kingdom, it was technically at war as of Britain's declaration, this is an acknowledgement of the declaration of war by Britain (due to coercion by an advisor), Oman, under British influence, is coerced to participate in the war effort	Declaration Declaration Declaration
1939-09-17	Soviet Union	Poland	A[3][6]		Invasion
1939-11-09	Kuwait	Germany	W		Kuwaiti Declaration of War
1939-11-30	Soviet Union	Finland	A[3][6]		Invasion
1940-04-09	Germany	Denmark  Norway	A[3]		Invasion of Denmark Invasion of Norway
1940-04-12	United Kingdom	Faroe Islands	A[6]		Invasion
1940-05-10	Germany	Belgium  Netherlands	A/W[3][6]	Date of the German offensive in the West, W from Belgium and the Netherlands.[10]	Belgium Netherlands
1940-05-10	Germany	Luxembourg	A[6]		Luxembourg
1940-05-10	United Kingdom	Iceland	A		Invasion
1940-06-10	Italy	France  United Kingdom	W[3][6]		France and the UK
1940-06-10	Canada	Italy	W[3]		Declaration
1940-06-11	South Africa  Australia  New Zealand France	Italy	W[3]		South Africa Australia New Zealand France
1940-07-03	United Kingdom	Vichy France	A	Vichy France cuts off diplomatic relations with the United Kingdom on 8 July 1940.	Attack
1940-09-09	Italy	Egypt	A	Egypt never formally declared war on Italy.	Invasion
1940-09-22	Japan	Vichy France	A	Japanese troops occupy French Indochina	Invasion
1940-09-23	Free France  United Kingdom  Australia	Vichy France	A		Invasion
1940-10-??	Thailand	Vichy France	A		Franco-Thai War
1940-10-28	Italy	Greece	U[6]	Italy invades Greece	Invasion
1940-11-23	Belgium (in-exile)	Italy	W[11]		Declaration
1941-02-05	Free France	Italy	A		Invasion
1941-04-06	Germany	Greece	W[3][6]		Invasion
1941-04-06	Germany Italy	Yugoslavia	A[3][6]		Invasion
1941-04-07	Yugoslavia	Hungary	A[12]	After the German invasion, bombing of Hungarian locations	Invasion
1941-04-14	Germany	Egypt	A	Egypt did not formally declare war until 1945.	Invasion
1941-05-02	United Kingdom	Iraq	A		Invasion
1941-06-08	Free France	Germany	A	[citation needed]	Invasion
1941-06-22	Germany Italy	Soviet Union	W[6]	A timed declaration of war was given by Germany at the time of the attack[13]	Invasion
1941-06-22/24	Romania	Soviet Union	A	On June 22, Romanian leader Ion Antonescu declared a "holy war" to reclaim ancestral lands and against Bolshevism in an appeal to the nation.[14] The Romanian army began limited military operations. On June 24, Romania officially declared war on the Soviet Union.[14]
1941-06-22	Tuva	Germany	W	Tuva was a client state of the Soviet Union. Part of the USSR from 1944.
1941-06-24	Bulgaria	Greece Yugoslavia	A[6]	Bulgaria declares war on Greece and Yugoslavia.
1941-06-25	Finland	Soviet Union	W	After the bombing of several Finnish locations, Finland recognized a state of war with the Soviet Union; third war between these nations.	Continuation War
1941-06-27	Hungary	Soviet Union	C[12]	After the bombing of several Hungarian locations, the Hungarian military concluded a Soviet attack, the Government had decided the two countries were already belligerent, without the consent of the Parliament, in absence of the Regent	Invasion
1941-08-25	Soviet Union  United Kingdom  Australia	Iran	A		Invasion
1941-12-05	United Kingdom  India	Finland Romania Hungary	W[15]		Declaration
1941-12-06	Finland Romania	United Kingdom	W[6]		Declaration
1941-12-07	United Kingdom	Hungary	W[16]	UK declaration of war from 1941-12-05 entered into force on 1941-12-07 1 minute after midnight	Declaration
1941-12-07	Japan	United States  United Kingdom  Thailand	A	A formal message breaking off diplomatic talks was sent before but arrived after the attacks began, but this was not a declaration of war.[17] See Attack on Pearl Harbor § Japanese declaration of war	Declaration published after: Attack on Pearl Harbor Invasion of Hong Kong Invasion of Thailand
1941-12-07	Canada  Australia  New Zealand	Finland Romania Hungary	W[6]		Declaration
1941-12-07	Canada  Panama Yugoslavia (in-exile)	Japan	W[6]		Declaration
1941-12-08	United Kingdom  United States  Canada  Australia  Costa Rica  Dominican Republic  El Salvador Free France  Guatemala Greece (in-exile) Haiti  Honduras Netherlands (in-exile)  New Zealand  Nicaragua  Philippines Tonga[9]	Japan	W[6]	After the attack on Pearl Harbor, many countries declared a formal state of war on Japan	British declaration United States declaration Canadian declaration Australian declaration Costa Rican declaration Dominican Republic's declaration Guatemalan declaration Salvadoran declaration Haitian declaration Honduran declaration Dutch declaration New Zealand's declaration Nicaraguan declaration Philippine declaration Tongan declaration
1941-12-08	South Africa	Japan  Finland Romania Hungary	W[6]		South African declaration
1941-12-08	Mongolia	Germany	W[citation needed]		Mongolian declaration
1941-12-08	Manchukuo	United States	W[6]		Manchukuo declaration
1941-12-08	Japan	British Malaya	A		Invasion of Malaya Attack on Singapore
1941-12-09	Cuba	Japan	W[6]
1941-12-09	China	Germany Italy  Japan	W[6]	China and Japan had been at undeclared war since 1937	Second Sino-Japanese war Declaration
1941-12-11	Germany Italy	United States	W[3][6]		German declaration Italian declaration
1941-12-11	United States Cuba  Costa Rica  Dominican Republic  Guatemala  Nicaragua	Germany Italy	W[6]		Germany Italy
1941-12-11	Netherlands (in-exile)	Italy	W[6]		Declaration
1941-12-11	Poland (in-exile)	Japan	W[3]	Japan rejected declaration of War. Prime Minister Hideki Tōjō's answer was: "We don't accept the Polish declaration of war. The Poles, fighting for their freedom, declared war under the British pressure"[citation needed].
1941-12-12	Romania Bulgaria  Slovakia	United States  United Kingdom	W[6]		Romanian declaration Bulgarian declaration
1941-12-12	Haiti  El Salvador  Panama	Germany Italy	W[6]		Declaration
1941-12-12	Australia  Netherlands	Portugal	A	Portugal maintained neutrality throughout World War II.	Invasion
1941-12-13	United Kingdom  New Zealand South Africa	Bulgaria	W[6]		British declaration New Zealand's declaration South African declaration
1941-12-13	Honduras	Germany Italy	W[6]		Declaration
1941-12-13	Italy	Cuba  Guatemala	W[6]		Declaration
1941-12-14	Independent State of Croatia	United States  United Kingdom	W[6]		Declaration
1941-12-15	Hungary	United States	C[18]	The Prime Minister informed the U.S. ambassador without approval of the Parliament and the Regent, but initially denied it would mean "war" in fact, however two days later he declared it means the two countries became belligerent. As the ambassador refused to accept the verbal form of this act, the next day the Prime Minister in written reinforced it.	Declaration
1941-12-16	Czechoslovakia (in-exile)	Germany Italy  Finland Romania Hungary Japan Bulgaria Independent State of Croatia  Slovakia	W[6]	Czechoslovakia declares war on all countries at war with the United States of America, Great Britain and the Union of Soviet Socialist Republics.	Declaration
1941-12-16	Japan	Sarawak North Borneo Brunei	A		Invasion
1941-12-17	Albania	United States	W		Declaration
1941-12-19	Nicaragua	Romania Hungary Bulgaria	W[6]		Declaration
1941-12-19	Romania	Nicaragua	W[19]	Out of the Axis countries only Romania reciprocated, declaring war on Nicaragua on the same day	Declaration
1941-12-20	Belgium (in-exile)	Japan	W		Declaration
1941-12-24	Haiti	Romania Hungary Bulgaria	W[6]		Declaration
1941-12-24	Romania	Haiti	W[20]	Out of the Axis countries only Romania reciprocated, declaring war on Haiti on the same day	Declaration
1941-12-25	Greece (in-exile)	Japan	W		Declaration
1942-01-01	United Nations	Axis Powers	W	Declared during Arcadia Conference	Declaration
1942-01-06	Australia	Bulgaria	W		Declaration
1942-01-25	Thailand	United Kingdom  United States	A		Declaration
1942-01-25	United Kingdom  New Zealand South Africa	Thailand	W		British declaration New Zealand's declaration South African declaration
1942-02-19	Japan	Portugal	A	Portugal maintained neutrality throughout World War II.	Invasion
1942-03-02	Australia	Thailand	W[6]		Australian declaration
1942-05-05	South Africa Netherlands (in-exile) Poland	Vichy France	A		Invasion
1942-05-05	United Kingdom Northern Rhodesia Southern Rhodesia South Africa Tanganyika Belgian Congo Poland (in-exile)	Vichy France  Japan	A		Invasion
1942-05-22	Mexico	Germany Italy  Japan	W[6]		Declaration
1942-06-05	United States	Hungary Romania Bulgaria	W[12]		United States Declaration of War upon Hungary United States Declaration of War upon Romania United States declaration of war upon Bulgaria
1942-06-13	Iroquois Confederacy	Germany Italy  Japan	W	Having never made peace with Germany from the First World War, Haudenosaunee became the only Native American state to officially declare war on the Axis powers separately from the United States (other Native American nations issued declarations or declared war de facto alongside the United States as their tribal citizens enlisted in the Armed Forces[21]).
1942-08-22	Brazil	Germany Italy	W		Declaration
1942-11-08	United States Canada	Vichy France	A		Invasion
1942-11-10	Germany Italy	Vichy France	A		Invasion
1942-11-12	Germany	Tunisia	A	German invasion via airlifting several divisions in reaction to Operation Torch, swiftly occupying Tunis and the eastern part of the country, and capturing the western portions after French Tunisian resistance before the allies reached the Tunisian border. In the resulting Tunisian campaign, the Allies finally defeated the Axis forces in Africa.	Invasion
1942-12-14	Ethiopia	Germany Italy  Japan	W	On 3 October 1935, Italy invaded Ethiopia without a formal declaration of war. In response to the Italian invasion, Ethiopia declared war on Italy. Most of Ethiopia was occupied by Italy in 1936, however parts of Ethiopia remained under the control of the Ethiopian Patriots Movement, which begun its guerrilla war against the occupying Italian forces the day Addis Ababa fell in May 1936. In May 1941, Addis Ababa was liberated by the Gideon Force, restoring sovereignty to Ethiopia.	Second Italo-Ethiopian War Declaration
1943-01-09	Reorganized National Government of China	United States  United Kingdom	W[6]		Declaration
1943-01-17	Iraq	Germany Italy  Japan	W[6]		Declaration
1943-01-20	Chile	Germany Italy	W	Chile cuts off diplomatic relations with the German Reich and the Kingdom of Italy on 20 January 1943.	Declaration
1943-04-07	Bolivia	Germany Italy  Japan Other axis nations	W	Bolivia officially joined the Allies on 7 April 1943. Shortly after war was declared, the President, Enrique Peñaranda, was overthrown in a coup. Bolivian mines supplied needed tin to the Allies, but no troops or warplanes were sent overseas. Bolivians remained confident their geographic isolation would protect them from the war.	Declaration
1943-08-01	State of Burma	United States  United Kingdom	W[6]		Declaration
1943-09-08	Germany	Italy Albania	A	After Italy's capitulation to the Allied powers, Germany swiftly invaded both Italy and Italian-controlled territories (such as Albania, an Italian satellite state) to preempt a possible Allied intervention.	Operation Achse, German occupation of Albania
1943-09-09	Iran	Germany	W		Declaration
1943-10-13	Italy	Germany	W[3]	Italy had changed sides after the fall of Mussolini. The Declaration of War was given by Pietro Badoglio to the German ambassador in Madrid.[3]	Declaration
1943-11-26	Colombia	Germany	W	See Colombia during World War II	Declaration
1944-01-17	Free France	Italian Social Republic	A		Invasion
1944-01-27	Liberia	Germany  Japan	W		Declaration
1944-06-06	France	Germany	A		Invasion
1944-07-25	France	Japan	A		Invasion
1944-08-25	Romania	Germany	W[6]	Romania switched sides	Declaration
1944-09-05	Soviet Union	Bulgaria	W		Declaration
1944-09-07	Hungary	Romania	W[6]		Declaration
1944-09-07	Romania	Hungary	W[6]		Declaration
1944-09-08	Bulgaria	Germany	W [citation needed]	Bulgaria switched sides	Declaration
1944-09-15	Germany	Finland	A		Lapland War
1944-09-21	San Marino	Germany Italian Social Republic	W[22]		Battle of San Marino
1944-09-23	Second Philippine Republic	United States  United Kingdom	W[23]		Philippine declaration
1944-12-28	Hungary	Germany	W[24]	The Provisional National Government, which had been established under Soviet protection in the city of Debrecen, declared war on Germany. The German-backed Arrow Cross regime was still at war with the Soviet Union and its troops were still in action. As it had not switched sides of its own accord, Hungary was never recognized as having defected to the Allies.	Hungarian declaration
1945-02-02	Ecuador	Germany  Japan	W		Ecuadorian declaration
1945-02-07	Paraguay	Germany  Japan	W		Paraguayan declaration
1945-02-12	Peru	Germany  Japan	W[6]		Declaration
1945-02-15	Venezuela  Uruguay	Germany  Japan	W		Venezuelan declaration Uruguayan declaration
1945-02-23	Turkey	Germany  Japan	W		Declaration
1945-02-24	Egypt	Germany  Japan	W		Declaration
1945-02-26	Syria  Lebanon	Germany  Japan	W		Declaration
1945-02-28	Saudi Arabia	Germany	W		Declaration
1945-03-01	Saudi Arabia	Japan	W[6]		Declaration
1945-03-01	Iran	Japan	W[6]	Iran declares war on Japan retroactive to the previous day (Feb. 28, 1945)	Declaration
1945-03-03	Finland	Germany	W	Finland declares war on Germany retroactive to Sept. 15, 1944 following terms of 1944 Moscow Armistice	Lapland War
1945-03-07	Romania	Japan	W[6]		Declaration
1945-03-27	Argentina	Germany  Japan	W		Argentinean declaration
1945-04-11	Chile	Japan	W		Declaration
1945-06-07	Brazil	Japan	W[6]		Declaration
1945-07-09	Norway (in-exile)	Japan	W	Norwegian government-in-exile announces that it had declared war on Japan on December 7, 1941	Declaration[25]
1945-07-14	Italy	Japan	W		Declaration
1945-08-08	Soviet Union	Japan	W[3]	Last outbreak of war during the entire Second World War.	Soviet–Japanese War
1945-08-10	Mongolia	Japan	W[26]	W (de jure) A (de facto 1945-08-09) War declared 24 hours after crossing the border with Soviet troops	Soviet Invasion of Manchuria Mongolia in World War II

• 
  German ambassador, Hans-Adolf von Moltke, Polish leader Józef Piłsudski, German propaganda minister Joseph Goebbels and Józef Beck, Polish Foreign minister meeting in Warsaw on June 15, 1934, five months after signing the Polish-German Non-Aggression Pact.
• 
  British Prime Minister Neville Chamberlain, landing at Heston aerodrome on 30 September 1938 after his meeting with Hitler at Munich. In his hand he holds the peace agreement between Britain and Germany.
• 
  Common parade of German Wehrmacht and Soviet Red Army on September 23, 1939 in Brest, Eastern Poland at the end of the Poland Campaign. In the center is Major General Heinz Guderian; and on the right is Brigadier General Semyon Krivoshein.
• 
  Counsellor of state J.K. Paasikivi and his team arriving from Moscow for the first round of negotiations on 16 October 1939. From left, minister Aarno Yrjö-Koskinen, J.K. Paasikivi, chief of staff Johan Nykopp and colonel Aladár Paasonen.
• 
  German paratroopers taking the Greek island of Crete, May 1941.
• 
  Photograph from a Japanese plane of Battleship Row at the beginning of the surprise Attack on Pearl Harbor. The explosion in the center is a torpedo strike on the USS Oklahoma. Two attacking Japanese planes can be seen: one over the USS Neosho and one over the Naval Yard.

• Diplomatic history of World War II
• Allies of World War II
• Axis powers: German Instrument of Surrender and Surrender of Japan
• Declarations of war during World War I

• Boog, Horst; Förster, Jürgen; Hoffmann, Joachim; Klink, Ernst; Müller, Rolf-Dieter; Ueberschär, Gerd R. (1998). Germany and the Second World War. Vol. 4. Oxford: Clarendon Press.
• Prange, Gordon William; Goldstein, Donald M.; Dillon, Katherine V. (1981). At Dawn We Slept: The Untold Story of Pearl Harbor. McGraw-Hill. ISBN 0-07-050669-8.