List of All War Declarations and Other Outbreaks of Hostilities During World War II

[Thousands of photographs and illustrations of World War II are in my World War II In View Flickr collection.]  

Regarding type of war outbreak (fourth column):

A = Attack without a declaration of war

U = State of war emerged through ultimatum

WD = State of war emerged after formal declaration of war

D = Diplomatic breakdown leading to a state of war.

In some cases a diplomatic breakdown later led to a state of war. Such cases are mentioned in the comments.

Date	Attacking Nation(s)	Attacked Nation(s)	Type	Comments
1939-09-01	Germany	Poland	A
1939-09-03	United Kingdom, France	Germany	U
1939-09-03	Australia, New Zealand	Germany	WD
1939-09-06	South Africa	Germany	WD
1939-09-10	Canada	Germany	WD
1939-09-17	Soviet Union	Poland	A
1939-11-30	Soviet Union	Finland	A	Diplomatic breakdown day before
1940-04-09	Germany	Denmark, Norway	A
1940-05-15	Germany	Belgium, Netherlands	WD	The German offensive in western Europe
1940-06-10	Italy	France, United Kingdom	WD	At a time when France already was about to fall
1940-06-10	Canada	Italy	WD
1940-06-11	South Africa, Australia, New Zealand	Italy	WD
1940-06-12	Egypt	Italy	D
1940-07-04	United Kingdom	France*	A	Vichy France Navy and colonies were attacked by UK, but no war was declared
1940-10-28	Italy	Greece	U
1941-04-06	Germany	Greece	WD
1941-04-06	Germany, Bulgaria	Yugoslavia	A
1941-04-06	Italy	Yugoslavia	WD
1941-04-10	Hungary	Yugoslavia	A
1941-04-23	Greece	Bulgaria	D
1941-06-22	Germany*, Italy, Romania	Soviet Union	WD	*The German declaration of war was given at the time of the attack
1941-06-24	Denmark	Soviet Union	D	Denmark was occupied by Germany
1941-06-25	Finland	Soviet Union	A	Second war between these nations.
1941-06-27	Hungary	Soviet Union	WD	Diplomatic breakdown 1941-06-24
1941-06-30	France	Soviet Union	D
1941-12-07	United Kingdom	Romania, Hungary, Finland	U	Diplomatic breakdowns 1941-02-11,1941-04-07 and 1941-08-01
1941-12-07	Japan	United States	A	WD came the day after.
1941-12-08	Japan	United Kingdom	WD
1941-12-08	Canada, South Africa	Japan	WD
1941-12-08	China	Germany*, Italy*, Japan	WD	*Diplomatic breakdown 1941-07-02
1941-12-09	Australia, New Zealand	Japan	WD
1941-12-11	Germany, Italy	United States	WD
1941-12-12	Romania	United States	WD
1941-12-13	Bulgaria	United Kingdom, United States	WD
1941-12-13	Hungary	United States	WD
1942-01-24	United States	Denmark	D
1942-05-28	Mexico	Germany, Italy, Japan	WD	Diplomatic breakdowns in all three cases 1941
1942-08-22	Brazil	Germany, Italy	WD	Diplomatic breakdowns 1942-01-20 and 1942-01-28
1942-11-09	France	United States	D
1943-01-20	Chile	Germany, Japan, Italy	D
1943-09-09	Iran	Germany	WD	Diplomatic breakdown in 1941
1943-10-13	Italy	Germany	WD	After the fall of Mussolini, Italy changed side
1944-01-10	Argentina	Germany, Japan	D
1944-06-30	United States	Finland	D
1944-08-04	Turkey	Germany	D	Turkey declared war on Germany on 23 Feb. 1945; a state of war against Germany existed from this date.
1944-08-23	Romania	Germany	WD	Like Italy, Romania also changed side.
1944-09-05	Soviet Union	Bulgaria	WD
1944-09-07	Bulgaria	Germany	D
1945-02-24	Egypt	Germany*, Japan	WD	*Diplomatic breakdown already 1939
1945	Argentina, Paraguay, Peru, Venezuela, Uruguay, Syria, and Saudi Arabia	Germany	WD	Needed a declaration to be eligible to join United Nations
1945-04-03	Finland	Germany	WD	Diplomatic breakdown in 1944, last outbreak in Europe.
1945-07-06	Brazil	Japan	WD
1945-07-17	Italy	Japan	WD
1945-08-08	Soviet Union	Japan	WD	Last outbreak of war during the Second World War.