| Entry | End | Years | Months | Days | Total | Combat | |
| Austria-Hungary | 28 July, 1914 | 4 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| Germany | 1 August, 1914 | 11 November, 1918 | 4 | 3 | 11 | 1,564 | Y |
| Bulgaria | 14 October, 1914 | 29 September, 1918 | 3 | 11 | 17 | 1,447 | Y |
| Ottoman Empire | 29 October, 1914 | 30 October, 1918 | 3 | 11 | 2 | 1,433 | Y |
| Entry | End | Years | Months | Days | Total | Combat | |
| Serbia | 28 July, 1914 | 11 November, 1918 | 4 | 3 | 15 | 1,568 | Y |
| Russia | 1 August, 1914 | 3 March, 1918 | 3 | 7 | 3 | 1,311 | Y |
| France | 3 August, 1914 | 11 November, 1918 | 4 | 3 | 9 | 1,562 | Y |
| Belgium | 4 August, 1914 | 11 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| United Kingdom | 4 August, 1914 | 11 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| Australia | 4 August, 1914 | 11 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| Canada | 4 August, 1914 | 11 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| New Zealand | 4 August, 1914 | 11 November, 1918 | 4 | 3 | 8 | 1,561 | Y |
| Montenegro | 6 August, 1914 | 17 January, 1916 | 1 | 5 | 12 | 530 | Y |
| Japan | 23 August, 1914 | 11 November, 1918 | 4 | 2 | 20 | 1,542 | Y |
| Portugal | 23 November, 1914 | 11 November, 1918 | 3 | 11 | 20 | 1,450 | Y |
| Italy | 23 May, 1915 | 11 November, 1918 | 3 | 5 | 20 | 1,269 | Y |
| San Marino | 3 June, 1915 | 4 November, 1918 | 3 | 5 | 2 | 1,251 | - |
| Romania | 27 August, 1916 | 7 May, 1918 | 1 | 8 | 11 | 619 | Y |
| United States | 6 April, 1917 | 11 November, 1918 | 1 | 7 | 6 | 585 | Y |
| Panama | 7 April, 1917 | 11 November, 1918 | 1 | 7 | 5 | 584 | - |
| Cuba | 7 April, 1917 | 11 November, 1918 | 1 | 7 | 5 | 584 | - |
| Greece | 28 June, 1917 | 11 November, 1918 | 1 | 4 | 15 | 502 | Y |
| Siam | 22 July, 1917 | 11 November, 1918 | 1 | 3 | 21 | 478 | - |
| Liberia | 4 August, 1917 | 11 November, 1918 | 1 | 3 | 8 | 465 | - |
| China | 14 August, 1917 | 11 November, 1918 | 1 | 2 | 29 | 455 | - |
| Brazil | 26 October, 1917 | 11 November, 1918 | 1 | 0 | 17 | 382 | - |
| Guatemala | 21 April 1918 | 11 November, 1918 | - | 6 | 22 | 205 | - |
| Nicaragua | 6 May, 1918 | 11 November, 1918 | - | 6 | 6 | 190 | - |
| Costa Rica | 23 May, 1918 | 11 November, 1918 | - | 5 | 20 | 173 | - |
| Haiti | 12 July, 1918 | 11 November, 1918 | - | 4 | 0 | 123 | - |
| Honduras | 19 July, 1918 | 11 November, 1918 | - | 3 | 24 | 116 | - |
Notes:
(1) Total days count includes end date.
Source: Adapted from The War With Germany: A Statistical Survey.