| Entry | End | Years | Months | Days | Total | Combat | |
| Germany | 1 September, 1939 | 8 May, 1945 | 5 | 8 | 8 | 2,077 | Y |
| Italy | 1 June, 1940 | 8 September, 1943 | 3 | 3 | 8 | 1,195 | Y |
| Hungary | 10 April, 1941 | 20 January, 1945 | 3 | 9 | 11 | 1,382 | Y |
| Bulgaria | 24 April, 1941 | 8 September, 1944 | 3 | 4 | 16 | 1,234 | Y |
| Romania | 22 June, 1941 | 24 August, 1944 | 3 | 2 | 3 | 1,160 | Y |
| Finland | 25 June, 1941 | 15 September, 1944 | 3 | 2 | 22 | 1,179 | Y |
| Slovakia | 1 December, 1941 | 4 April, 1945 | 3 | 4 | 4 | 1,221 | Y |
| Japan | 7 December, 1941 | 15 August, 1945 | 3 | 8 | 9 | 1,348 | Y |
| Croatia | 14 December, 1941 | 8 May, 1945 | 3 | 4 | 25 | 1,242 | Y |
| Albania | 17 December, 1941 | 29 November, 1944 | 2 | 11 | 13 | 1,079 | Y |
| Thailand | 25 January, 1942 | 15 August, 1945 | 3 | 6 | 22 | 1,299 | Y |
| Entry | End | Years | Months | Days | Total | Combat | |
| Poland | 1 September, 1939 | 27 September, 1939 | - | - | 28 | 28 | Y |
| France | 3 September, 1939 | 25 June, 1940 | - | 9 | 23 | 297 | Y |
| United Kingdom | 3 September, 1939 | 15 August, 1945 | 5 | 11 | 13 | 2,174 | Y |
| Australia | 3 September, 1939 | 15 August, 1945 | 5 | 11 | 13 | 2,174 | Y |
| India | 3 September, 1939 | 15 August, 1945 | 5 | 11 | 13 | 2,174 | Y |
| New Zealand | 3 September, 1939 | 15 August, 1945 | 5 | 11 | 13 | 2,174 | Y |
| South Africa | 3 September, 1939 | 15 August, 1945 | 5 | 11 | 13 | 2,174 | Y |
| Canada | 10 September, 1939 | 15 August, 1945 | 5 | 11 | 6 | 2,167 | Y |
| Denmark | 9 April, 1940 | 9 April, 1940 | - | - | 1 | 1 | Y |
| Norway | 9 April, 1940 | 10 June, 1940 | - | 2 | 2 | 63 | Y |
| Belgium | 10 May, 1940 | 28 May, 1940 | - | - | 19 | 19 | Y |
| Luxembourg | 10 May, 1940 | 10 May, 1940 | - | - | 1 | 1 | Y |
| Netherlands | 10 May, 1940 | 14 May, 1940 | - | - | 5 | 5 | Y |
| Greece | 28 October, 1940 | 1 June, 1941 | - | 7 | 5 | 217 | Y |
| Yugoslavia | 6 April, 1941 | 17 April, 1941 | - | - | 12 | 12 | Y |
| U.S.S.R. | 22 June, 1941 | 15 August, 1945 | 4 | 1 | 25 | 1,516 | Y |
| United States | 7 December, 1941 | 15 August, 1945 | 3 | 8 | 9 | 1,348 | Y |
| Panama | 7 December, 1941 | 15 August, 1945 | 3 | 8 | 9 | 1,348 | - |
| Costa Rica | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | - |
| Dominican Republic | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | - |
| Guatemala | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | - |
| Honduras | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | - |
| Nicaragua | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | - |
| Philippines | 8 December, 1941 | 15 August, 1945 | 3 | 8 | 8 | 1,347 | Y |
| China | 9 December, 1941 | 15 August, 1945 | 3 | 8 | 7 | 1,346 | Y |
| Brazil | 2 May, 1942 | 15 August, 1945 | 3 | 3 | 14 | 1,202 | Y |
| Mexico | 22 May, 1942 | 15 August, 1945 | 3 | 2 | 25 | 1,182 | Y |
| Ethiopia | 14 December, 1942 | 15 August, 1945 | 2 | 8 | 2 | 976 | - |
| Iraq | 16 January, 1943 | 15 August, 1945 | 2 | 7 | 0 | 943 | - |
| Bolivia | 7 April, 1943 | 15 August, 1945 | 2 | 4 | 9 | 862 | - |
| Iran | 9 September, 1943 | 8 May, 1945 | 1 | 8 | 0 | 608 | - |
| Italy | 13 October, 1943 | 8 May, 1945 | 1 | 6 | 26 | 574 | Y |
| Colombia | 26 November, 1943 | 8 May, 1945 | 1 | 5 | 13 | 530 | - |
| Liberia | 27 January, 1944 | 15 August, 1945 | 1 | 6 | 20 | 567 | - |
| Romania | 24 August, 1944 | 8 May, 1945 | - | 8 | 15 | 258 | Y |
| Bulgaria | 8 September, 1944 | 8 May, 1945 | - | 8 | 1 | 243 | Y |
| France (Provisional) | 9 September, 1944 | 8 May, 1945 | - | 8 | 0 | 242 | Y |
| Finland | 15 September, 1944 | 8 May, 1945 | - | 7 | 24 | 236 | Y |
| Hungary | 20 January, 1945 | 8 May, 1945 | - | 3 | 19 | 109 | Y |
| Ecuador | 2 February, 1945 | 15 August, 1945 | - | 6 | 14 | 195 | - |
| Paraguay | 8 February, 1945 | 15 August, 1945 | - | 6 | 8 | 189 | - |
| Peru | 11 February, 1945 | 15 August, 1945 | - | 6 | 5 | 186 | - |
| Chile | 12 February, 1945 | 15 August, 1945 | - | 6 | 4 | 185 | - |
| Venezuela | 14 February, 1945 | 15 August, 1945 | - | 6 | 2 | 183 | - |
| Uruguay | 22 February, 1945 | 15 August, 1945 | - | 5 | 25 | 175 | - |
| Turkey | 23 February, 1945 | 15 August, 1945 | - | 5 | 24 | 174 | - |
| Egypt | 24 February, 1945 | 15 August, 1945 | - | 5 | 23 | 173 | - |
| Syria | 26 February, 1945 | 15 August, 1945 | - | 5 | 21 | 171 | - |
| Lebanon | 27 February, 1945 | 15 August, 1945 | - | 5 | 20 | 170 | - |
| Saudi Arabia | 1 March, 1945 | 15 August, 1945 | - | 5 | 18 | 168 | - |
| Argentina | 27 March, 1945 | 15 August, 1945 | - | 4 | 20 | 142 | - |
| Mongolia | 9 August, 1945 | 15 August, 1945 | - | - | 7 | 7 | Y |
Notes:
(1) Total days count includes end date.
(2) China began fighting Japanese on 7 July, 1937 (8 years, 1 month, 9 days; 2,962 total days).
(3) Italy, Finland, Romania, Bulgaria and Hungary switched sides between 1943-45.
(4) Total days count ignores breaks in conflict such as U.S.S.R. between V-E Day and entry into war against Japan (8/5/45 - 9/8/45). Numerous countries that had declared war on Germany did not declare war against Japan until after V-E Day, this break is also ignored.
(5) For countries occupied by Germany in 1939-41, end date is date of initial surrender even though many of them contributed "free" troops which continued fighting for Allies until V-E Day. French Provisional Government (GPRF) is included as it represents re-establishment of French government in its own territory.
Source: Adapated from Wikipedia; The World At War