Dates when the declarations became "official"--not the dates when a declaration was asked for or presented to governing bodies.
Poland Declared War on Germany at Midnight 31st August - 1st Sept 1939. [See the German White Book]
Great Britain on Germany ....................................September 3, 1939, 11 a.m.
France on Germany.............................................September 3, 1939, 5 p.m.
India on Germany................................................September 3, 1939
Australia on German..........................................September 3, 1939
New Zealand on Germany.....................................September 3, 1939
Union of South Africa on Germany..........................September 6, 1939
Canada on Germany............................................September 10, 1939
Norway and Germany.........................................No formal declaration of war
Belgium and Germany........................................No formal declaration of war
Luxembourg and Germany .................................No formal declaration of war
The Netherlands on Germany...............................May 10, 1940
Italy on France ...............................................June 10, 1940
Canada on Italy ...............................................June 10, 1940
New Zealand on Italy ........................................June 11, 1940
Australia on Italy.............................................June 11, 1940
Union of South Africa on Italy..............................June 11, 1940
Greece on Italy ...............................................October 28,1940
Germany and Greece ........................................No formal declaration of war
Germany on Yugoslavia.....................................April 6, 1941
Italy and Yugoslavia.........................................No formal declaration of war
Yugoslavia on Bulgaria......................................April 6, 1941
Yugoslavia on Hungary.......................................April 10, 1941
Bulgaria on Greece...........................................April 24, 1941
Bulgaria on Yugoslavia......................................April 24, 1941
Germany on USSR............................................June 22, 1941
Italy on USSR.................................................June 22, 1941
Rumania on USSR...........................................No formal declaration of war
Finland on USSR.............................................No formal declaration of war
Hungary on USSR...........................................June 27, 1941
Great Britain on Rumania ...............................December 7, 1941
Great Britain on Hungary................................December 7, 1941
Canada on Finland.........................................December 7, 1941
Canada on Rumania.......................................December 7, 1941
Canada on Hungary........................................December 7, 1941
Australia on Finland......................................December 8, 1941
Australia on Rumania....................................December 8, 1941
Australia on Hungara.....................................December 8, 1941
Union of South Africa on Finland.......................December 8, 1941
Union of South Africa on Rumania.....................December 8, 1941
Union of South Africa on Hungary.....................December 8, 1941
New Zealand on Finland..................................December 7, 1941
New Zealand on Hungary................................December 7, 1941
New Zealand on Rumania...............................December 7, 1941
Japan on the United States............................December 7, 1941
Japan on the British Empire...........................December 7, 1941
Great Britain on Japan.................................December 8, 1941
The United States on Japan...........................December 8, 1941
Canada on Japan.........................................December 8, 1941, as of December 7, 1941
Costa Rica on Japan.....................................December 8, 1941
Dominican Republic on Japan.........................December 8, 1941
Guatemala on Japan....................................December 8, 1941
Haiti on Japan............................................December 8, 1941
Honduras on Japan......................................December 8, 1941
El Salvador on Japan ...................................December 8, 1941
Panama on Japan........................................December 8, 1941
Cuba on Japan...........................................December 9, 1941
The Netherlands on Japan ............................December 8, 1941
The Netherlands Indies on Japan December 8, 1941
China on Japan December 9, 1941
China on Germany December 9, 1941
China on Italy December 9, 1941
Union of South Africa on Japan December 8, 1941
Australia on Japan December 8, 1941
Free France on Japan December 8, 1941
Germany on the United States December 11, 1941
Italy on the United States December 11, 1941
The United States on Germany December 11, 1941
The United States on Italy December 11, 1941
Costa Rica on Germany and Italy December 11, 1941
Guatemala on Germany and Italy December 11, 1941
Cuba on Germany and Italy December 11, 1941
Nicaragua on Germany, Italy and Japan December 11, 1941
Poland on Japan December 11, 1941
Dominican Republic on Germany and Italy December 11, 1941
Haiti on Germany and Italy December 12, 1941
Honduras on Germany and Italy December 12, 1941
El Salvador on Germany and Italy December 12, 1941
Panama on Germany and Italy December 12, 1941
Rumania on the United States December 12, 1941
Bulgaria on the United States December 13, 1941
Bulgaria on Great Britain December 13, 1941
Hungary on the United States December 13, 1941
Czechoslovakia on all countries that are in
a state of war with Great Britain, theUnited
States, or the USSR December 16, 1941
Albania on the United States December 17, 1941
Nicaragua on Bulgaria, Hungary, & Rumania December 20, 1941
Belgium on Japan December 20, 1941
Haiti on Bulgaria, Hungary, and Rumania December 24, 1941
Great Britain on Bulgaria December 27, 1941, as of December 13, 1941
The Netherlands on Italy December 30, 1941, as of December 11, 1941
Union of South Africa on Bulgaria December 31, 1941, as of December 13, 1941