congress (novanet answer)
War may be declared only by CONGRESS (:
Iran & Iraq may have declared war on each other during their gulf war in 1980-1988.
The british
In May 1756, Britain declared war on France in Europe.
On 13 May 1846.
ITALY DECLARED WAR ON AUSTRIA.
Germany declared war on the US, the US then declared war on Germany.
Congress declared War on 13 May 1846 after the Thornton Affair.
In what country? In the United States, only Congress may declare war.
I will only list the more important countries involved in the war because there were many minor countries that are of very little importance to the war.Austria-Hungary declared war on Serbia July 28 1914Russia declared war on Austria-Hungary July 30 1914Germany declared war on Russia August 1 1914Germany declared war on France August 3 1914Germany declared war on Belgium August 4 1914Britain declared war on Germany August 4 1914Montenegro declared war on Austria-Hungary August 5 1914Serbia declared war on Germany August 6 1914Montenegro declared war on August 9 1914France declared war on Austria-Hungary August 11 1914Britain declared war on Austria-Hungary August 12 1914Austria-Hungary declared war on Belgium August 22 1914Japan declared war on Germany August 23 1914Japan declared war on Austria-Hungary August 25 1914Russia declared war on Ottoman Empire November 1 1914Serbia declared war on Ottoman Empire November 2 1914Montenegro declared war on Ottoman Empire November 3 1914Britain declared war on Ottoman Empire November 5 1914France declared war on Ottoman Empire November 5 1914Italy declared war on Austria-Hungary May 23 1915Italy declared war on Ottoman Empire August 21 1915Bulgaria declared war on Serbia October 14 1915Britain declared war on Bulgaria October 15 1915Montenegro declared war on Bulgaria October 15 1915France declared war on Bulgaria October 16 1915Italy declared war on Bulgaria October 19 1915Russia declared war on Bulgaria October 19 1915Romania declared war on Austria-Hungary August 27 1916Italy declared war on Germany August 27 1916Germany declared war on Romania August 28 1916Ottoman Empire declared war on Romania August 20 1916Bulgaria declared war on Romania September 1 1916United States declared war on Germany April 6 1917United States declared war on Austria-Hungary December 7 1917The Armistice occurred November 18 1918Note: where Britain is listed, this includes declarations of war by British Dominions also(Canada, Australia, New Zealand, South Africa and India), as they were not completely independent during the First World War and couldn't conduct Foreign Policy themselves.
War was declared.
31 August 1939 Poland declared war on Germany 03 September 1939 Britain and France declared war on Germany 10 May 1940 the Netherlands declared war on Germany 06 April 1941 Germany declared war on Yugoslavia 16 December 1941 Czechoslovakia declared war on Germany 22 June 1942 Germany declared war on the Soviet Union I have found no declarations of war between Germany and the following countries which they invaded: Denmark (invaded April 1940) Norway (invaded April 1940) Belgium (invaded May 1940) Luxemburg (invaded May 1940) Greece (invaded April 1941)