This is as complete a list as I can create from various sources. I cannot find any mention of the United States declaring war on Turkey or Bulgaria
If there are any missing declarations or any leads on the missing dates, I would be grateful for details.
The list doesn't include any of the new states created as a result of the Russian & Austrian defeats (Yugoslavia, Ukraine, Transcaucasus etc)
The lay-out of this page was originally designed by
Nick Luft from data supplied by
Jon Casey.
| Country Declaring War | Recipient | Date Declaration Made |
| Austria-Hungary | Serbia | 28th July 1914 |
| Germany | Russia | 1st August 1914 |
| Germany | France | 3rd August 1914 |
| Germany | Belgium | 4th August 1914 |
| Britain | Germany | 4th August 1914 |
| Austria-Hungary | Russia | 6th August 1914 |
| Austria-Hungary | Belgium | ?? August 1914 |
| Montenegro | Austria-Hungary | 7th August 1914 |
| Germany | Serbia* | 9th August 1914 |
| Montenegro | Germany* | 9th August 1914 |
| France | Austria-Hungary | 10th August 1914 |
| Britain | Austria-Hungary | 12th August 1914 |
| Japan | Germany | 23rd August 1914 |
| Japan | Austria-Hungary | 26th August 1914 |
| Turkey | Britain, France, Russia | 5th November 1914# |
| Italy | Austria-Hungary | 23rd May 1915 |
| Italy | Turkey | 20th August 1915 |
| Bulgaria | Serbia | 12th October 1915 |
| Britain | Bulgaria | 14th October 1915 |
| France | Bulgaria | ?? October 1915 |
| Russia | Bulgaria | ?? October 1915 |
| Italy | Bulgaria | 20th October 1915 |
| Albania | Austria-Hungary* | ?? January 1916 |
| Germany | Portugal | 10th March 1916 |
| Austria-Hungary | Portugal* | ?? March 1916 |
| Rumania | Austria-Hungary | 27th August 1916 |
| Germany | Rumania | 27th August 1916 |
| Italy | Germany | 28th August 1916 |
| Turkey | Rumania | 30th August 1916 |
| Bulgaria | Rumania | 1st September 1916 |
| USA | Germany | 6th April 1917 |
| Cuba | Germany | 7th April 1917 |
| Panama | Germany | 7th April 1917 |
| Guatemala | Germany | 27th April 1917## |
| Nicaragua | Germany | 8th May 1917 |
| Greece | Central Powers | 30th June 1917 |
| Siam | Central Powers | 22nd July 1917 |
| Liberia | Germany | 7th August 1917 |
| China | Germany | 14th August 1917 |
| Brazil | Germany | 26th October 1917 |
| USA | Austria-Hungary | 7th December 1917 |
| Panama | Austria-Hungary | 10th December 1917 |
| Cuba | Austria-Hungary | 11th December 1917 |
| Costa Rica | Germany | 23rd May 1918 |
| Haiti | Germany | 12th July 1918 |
| Honduras | Germany | 19th July 1918 |
Notes:
Germany occupied Luxembourg on 1st August without a
declaration of war. * or possibly the other way round. # Hostilities actually began on 31st October ## Actually a breaking of diplomatic relations which Guatemala
subsequently decided to treat as a declaration of war.