According to the U.S State Department's list, the nations involved in the conflict made declarations of war as follows:
Aug. 28, 1914 Austria against Belgium
Aug. 27, 1914 Austria against Japan
Aug. 9, 1914 Austria against Montenegro
Aug. 6, 1914 Austria against Russia
July 28, 1914 Austria against Serbia
Aug. 4, 1914 Belgium against Germany
Oct. 26, 1917 Brazil against Germany
Oct. 14, 1915 Bulgaria against Serbia
Aug. 14, 1917 China against Austria
Aug. 14, 1917 China against Germany
May 23, 1918 Costa Rica against Germany
Apr. 7, 1917 Cuba against Germany
Dec. 16, 1917 Cuba against Austria-Hungary
Aug. 13, 1914 France against Austria
Oct. 16, 1915 France against Bulgaria
Aug. 3, 1914 France against Germany
Nov. 5, 1914 France against Turkey
Aug. 4, 1914 Germany against Belgium
Aug. 3, 1914 Germany against France
Mar. 9, 1916 Germany against Portugal
Sept. 14, 1916 Germany against Rumania
Aug. 1, 1914 Germany against Russia
Aug. 13, 1914 Great Britain against Austria
Oct. 15, 1915 Great Britain against Bulgaria
Aug. 4, 1914 Great Britain against Germany
Nov. 5, 1914 Great Britain against Turkey
Nov. 28, 1916 Greece (Provincial Government) against Bulgaria
July 2, 1917 Greece against Bulgaria
Nov. 28, 1916 Greece (Provincial Government) against Germany
July 02, 1917 Greece against Germany
Apr. 22, 1918 Guatemala against Austria-Hungary
Apr. 22, 1918 Guatemala against Germany
July 15, 1918 Haiti against Germany
July 19, 1918 Honduras against Germany
May 24, 1915 Italy against Austria
Oct. 19, 1915 Italy against Bulgaria
Aug. 28, 1916 Italy against Germany
Aug. 21, 1915 Italy against Turkey
Aug. 23, 1914 Japan against Germany
Aug. 4, 1917 Liberia against Germany
Aug. 8, 1914 Montenegro against Austria
Aug. 9, 1914 Montenegro against Germany
May 24, 1918 Nicaragua against Germany
Dec. 10, 1917 Panama against Austria
Apr. 7, 1917 Panama against Germany
Nov. 23, 1914 Portugal against Germany (authorizing intervention)
May 19, 1915 Portugal against Germany (granting military aid)
Aug. 27, 1916 Rumania against Austria (accepted by Austria's allies)
Oct. 19, 1915 Russia against Bulgaria
Aug. 7, 1914 Russia against Germany
Nov. 3, 1914 Russia against Turkey
May 24, 1915 San Marino against Austria
Oct. 16, 1915 Serbia against Bulgaria
Aug. 6, 1914 Serbia against Germany
Dec. 2, 1914 Serbia against Turkey
July 22, 1917 Siam against Austria
July 22, 1917 Siam against Germany
Nov. 23, 1914 Turkey against Allies
Aug. 29, 1916 Turkey against Rumania
Dec. 7, 1917 United States against Austria-Hungary
Apr. 6, 1917 United States against Germany
“Two things are necessary to the life of pure faith. The first is, that we behold God alone, under all the imperfect coverings that conceal him; the second is, constantly to have our souls kept in a state of dependence.”
–Fenelon