In all, 32 nations were listed as combatants in World War I - some in name only, however. The seemingly disproportionate number of Allied nations is misleading. Germany's military and technological might enabled the Central Powers* to fight on nearly even terms for four years. * NOTE: The Central Powers are indicated by the shaded areas in the table below.
| 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 | 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 |