The Entente or Allies 

France- August 1 1914

Russia- July 30 1914

Serbia- July 28 1914

British Empire and its colonies- August 4 1914

USA- April 6 1917

Japan- August 23 1914

Greece- 30 June 1917

Italy- April 26 1915

Belgium- August 3 1914

Romania- August 27 1916
 

Central Powers

German Empire- August 1 1914 

Austria Hungary- July 18 1914

Ottoman Empire- November 2 1914

Bulgaria- October 14 1915

​