Combatants

The following countries were combatants in World War I. The original “Central Powers” in August 1914 were Austria-Hungary, Germany, and the Ottoman Empire (Turkey); Bulgaria joined the Central Powers in 1915.

Country Date Event
Austria-Hungary July 28, 1914 Declared war on Serbia
August 5, 1914 Declared war on Russia
August 28, 1914 Declared war on Belgium
March 15, 1916 Declared war on Portugal
Belgium August 3-4, 1914 Invaded by Germany
Bolivia April 13, 1917 Broke relations with Germany
Brazil April 11, 1917 Broke relations with Germany
October 26, 1917 Declared war on Germany
Bulgaria October 14, 1915 Declared war on Serbia
September 1, 1916 Declared war on Romania
China March 14, 1917 Broke relations with German
August 14, 1917 Declared war on Germany
August 14, 1917 Declared war on Austria-Hungary
Costa Rica September 21, 1917 Broke relations with Germany
May 23, 1918 Declared war on Germany
Cuba April 7, 1917 Declared war on Germany
Ecuador December 8, 1917 Broke relations with Germany
France August 12, 1914 Declared war on Austria-Hungary
August 23, 1914 Invaded by Germany
November 5, 1914 Declared war on Turkey
October 16, 1915 Declared war on Bulgaria
Germany August 1, 1914 Declared war on Russia
August 3, 1914 Declared war on France
August 4, 1914 Declared war on Belgium
March 9, 1916 Declared war on Portugal
Great Britain August 4, 1914 Declared war on Germany
August 12, 1914 Declared war on Austria-Hungary
November 5, 1914 Declared war on Turkey
October 15, 1915 Declared war on Bulgaria
Greece June 27, 1917 Declared war on Austria-Hungary, Bulgaria, Germany and Turkey
Guatemala April 23, 1918 Declared war on Germany
Haiti July 12, 1918 Declared war on Germany
Honduras July 19, 1918 Declared war on Germany
Italy May 23, 1915 Declared war on Austria-Hungary
August 21, 1915 Declared war on Turkey
October 19, 1915 Declared war on Bulgaria
August 28, 1916 Declared war on Germany
Japan August 23, 1914 Declared war on Germany
August 25, 1914 Declared war on Austria-Hungary
Liberia August 4, 1914 Declared war on Germany
Montenegro August 5, 1914 Declared war on Austria-Hungary
August 8, 1914 Declared war on Germany
October 15, 1915 Declared war on Bulgaria
Nicaragua May 8, 1918 Declared war on Germany and Austria-Hungary
Panama April 7, 1917 Declared war on Germany
December 10, 1917 Declared war on Austria-Hungary
Peru October 6, 1917 Broke relations with Germany
Portugal March 9, 1916 Object of German war declaration
March 15, 1916 Object of Austro-Hungarian war declaration
Romania August 27, 1916 Declared war on Austria-Hungary
May 7, 1918 Surrendered to Central Powers (Treaty of Bucharest)
November 10, 1918 Resumed hostilities against Central Powers
Russia November 2, 1914 Declared war on Turkey
October 19, 1915 Declared war on Bulgaria
San Marino June 3, 1915 Declared war on Austria-Hungary
Serbia August 6, 1914 Declared war on Germany
November 2, 1914 Declared war on Turkey
Siam July 22, 1917 Declared war on Germany and Austria-Hungary
Turkey

(Ottoman Empire)

August 30, 1916 Declared war on Romania
April 23, 1917 Broke relations with the US
United States April 6, 1917 Declared war on Germany
December 7, 1917 Declared war on Austria-Hungary
Uruguay October 7, 1917 Broke relations with Germany