Austria-Hungary

• Declared war on Serbia July 28, 1914
• Declared war on Russia Aug 6, 1914
• Declared war on Belgium Aug 28, 1914
• Declared war on Portugal March 15, 1916

Belgium

• Refused ultimatum demanding permission for German troops to cross Belgium Aug 2, 1914
• Invaded by Germany Aug 3-4, 1914

Bolivia

• Severed relations with Germany April 13, 1917

Brazil

• Severed relations with Germany April 11, 1917
• Declared war on Germany Oct 26, 1917

Bulgaria

• Declared war on Serbia Oct 14, 1915
• Declared war on Romania Sept 1, 1916

China

• Severed relations with German March 14, 1917
• Declared war on Germany Aug 14, 1917
• Declared war on Austria-Hungary Aug 14, 1917

Costa Rica

• Severed relations with Germany Sept 21, 1917
• Declared war on Germany May 23, 1918

Cuba

• Declared war on Germany April 7, 1917

Ecuador

• Severed relations with Germany Dec 8, 1917

France

• Invaded by Germany Aug 2, 1914
• Declared war on Austria-Hungary Aug 12, 1914
• Declared war on Turkey Nov 5, 1914
• Declared war on Bulgaria Oct 16, 1915

Germany

• Declared war on Russia Aug 1, 1914
• Declared war on France Aug 3, 1914
• Declared war on Belgium Aug 4, 1914
• Declared war on Portugal March 9, 1916

Great Britain

• Declared war on Germany Aug 4, 1914
• Declared war on Austria-Hungary Aug 12, 1914
• Declared war on Turkey Nov 5, 1914
• Declared war on Bulgaria Oct 15, 1915

Greece

• Declared war on Austria-Hungary, Bulgaria, Germany and Turkey June 27, 1917

Guatemala

• Declared war on Germany April 23, 1918

Haiti

• Declared war on Germany July 12, 1918

Honduras

• Declared war on Germany July 19, 1918

Italy

• Declared war on Austria-Hungary May 23, 1915
• Declared war on Turkey Aug 21, 1915
• Declared war on Bulgaria Oct 19, 1915
• Declared war on Germany Aug 28, 1916

Japan

• Declared war on Germany Aug 23, 1914
• Declared war on Austria-Hungary Aug 25, 1914

Liberia

• Declared war on Germany Aug 4, 1914

Montenegro

• Declared war on Austria-Hungary Aug 5, 1914
• Declared war on Germany Aug 8, 1914
• Declared war on Bulgaria Oct 15, 1915

Nicaragua

• Declared war on Germany and Austria-Hungary May 8, 1918

Panama

• Declared war on Germany April 7, 1917
• Declared war on Austria-Hungary Dec 10, 1917

Peru

• Severed relations with Germany Oct 6, 1917

Portugal

• See Germany and Austria-Hungary in this table

Romania

• Declared war on Austria-Hungary Aug 27, 1916
• Capitulated to Central Powers by Treaty of Bucharest May 7, 1918
• Re-entered the war on the side of the Allies Nov 10, 1918

Russia

• Declared war on Turkey Nov 2, 1914
• Declared war on Bulgaria Oct 19, 1915

San Marino

• Declared war on Austria-Hungary June 3, 1915

Serbia

• Declared war on Germany Aug 6, 1914
• Declared war on Turkey Nov 2, 1914

Siam

• Declared war on Germany and Austria-Hungary July 22, 1917

Turkey

• Declared war on Romania Aug 30, 1916
• Severed relations with the US April 23, 1917

United States

• Declared war on Germany April 6, 1917
• Declared war on Austria-Hungary Dec 7, 1917

Uruguay

• Severed relations with Germany Oct 7, 1917