Austria-Hungary declared war on Serbia. World War I began

The ‘Great War’, which began on 28 July 1914 with Austria-Hungary’s declaration of war with Serbia, was the first truly global war.  It began in Europe but quickly spread throughout the world.  Many countries became embroiled within the war’s first month; others joined in the ensuing four years, with Honduras announcing hostilities with Germany as late as 19 July 1918 (with the record going to Romania, who entered the war – albeit for the second time – one day before it finished, on 10 November 1918).

Detailed below is a list of the nations who formally declared hostilities during World War One, along with their date of entrance.  Nations of the British Empire, e.g. Australia, Canada and New Zealand, automatically entered the war with Britain’s decision to enter the fray on 4 August 1914.

Note that on numerous occasions hostilities were assumed without a formal declaration, e.g. Russia with Germany and Austria-Hungary in August 1914.

Australia
Entered war together with Britain on 4 August 1914

Austria-Hungary
Declared war with Serbia on 28 July 1914
Declared war with Russia on 6 August 1914
Declared war with Belgium on 28 August 1914
Declared war with Portugal on 15 March 1916

Belgium
Invaded by Germany on 3 August 1914

Bolivia
Severed relations with Germany on 13 April 1917

Brazil
Severed relations with Germany on 11 April 1917
Declared war with Germany on 26 October 1917

Bulgaria
Declared war with Serbia on 14 October 1915
Declared war with Romania on 1 September 1916

Canada
Entered war together with Britain on 4 August 1914

China
Severed relations with Germany on 14 March 1917
Declared war with Germany on 14 August 1917
Declared war with Austria-Hungary on 14 August 1917

Costa Rica
Severed relations with Germany on 21 September 1917
Declared war with Germany on 23 May 1918

Cuba
Declared war with Germany on 7 April 1917

Ecuador
Severed relations with Germany on 8 December 1917

France
Invaded by Germany on 2 August 1914
Declared war with Austria-Hungary on 12 August 1914
Declared war with Turkey on 5 November 1914
Declared war with Bulgaria on 16 October 1915

Germany
Declared war with Russia on 1 August 1914
Declared war with France on 3 August 1914
Declared war with Belgium on 4 August 1914
Declared war with Portugal on 9 March 1916

Greece
Declared war with Austria-Hungary on 27 June 1917
Declared war with Bulgaria on 27 June 1917
Declared war with Germany on 27 June 1917
Declared war with Turkey on 27 June 1917

Guatemala
Declared war with Germany on 23 April 1918

Haiti
Declared war with Germany on 12 July 1918

Honduras
Declared war with Germany on 19 July 1918

Italy
Declared war with Austria-Hungary on 23 May 1915
Declared war with Turkey on 21 August 1915
Declared war with Germany on 28 August 1915
Declared war with Bulgaria on 19 October 1915

Japan
Declared war with Germany on 23 August 1914
Declared war with Austria-Hungary on 25 August 1914

Liberia
Declared war with Germany on 4 August 1914

Montenegro
Declared war with Austria-Hungary on 5 August 1914
Declared war with Germany on 8 August 1914
Declared war with Bulgaria on 15 October 1915

New Zealand
Entered war together with Britain on 4 August 1914

Nicaragua
Declared war with Austria-Hungary on 8 May 1918
Declared war with Germany on 8 May 1918

Panama
Declared war with Germany on 7 April 1917
Declared war with Austria-Hungary on 10 December 1917

Peru
Severed relations with Germany on 6 October 1917

Portugal
Entered war against Germany on 9 March 1916
Entered war against Austria-Hungary on 15 March 1916

Romania
Declared war with Austria-Hungary on 27 August 1916
Exited war with Treaty of Bucharest on 7 May 1918
Re-entered the war on 10 November 1918

Russia
Declared war with Turkey on 2 November 1914
Declared war with Bulgaria on 19 October 1915

San Marino
Declared war with Austria-Hungary on 3 June 1915

Serbia
Declared war with Germany on 6 August 1914
Declared war with Turkey on 2 November 1914

Siam
Declared war with Austria-Hungary on 22 July 1917
Declared war with Germany on 22 July 1917

Turkey
Declared war with Romania on 30 August 1916
Severed relations with United States on 23 April 1917

United Kingdom
Declared war with Germany on 4 August 1914
Declared war with Austria-Hungary on 12 August 1914
Declared war with Turkey on 5 November 1914
Declared war with Bulgaria on 15 October 1915

United States of America
Declared war with Germany on 6 April 1917
Declared war with Austria-Hungary on 7 December 1917

Uruguay
Severed relations with Germany on 7 October 1917

Reference:
Funk & Wagnalls New Encyclopedia, Volume 27, Funk & Wagnall, 1983