I have wrote about what countries were involved in World War. 

Over twenty-seven countries were mentioned as the Allied and Associated Powers in the Treaty of Versailles.

Austria-Hungary : Austria-Hungary initiated World War I by declaring war on Serbia in July 1914.

Montenegro , Ottoman Empire , Japan , France and Colonies joined the war in 1914.

Bulgaria  , Italy, Germany , Portugal joined the war in 1915.

Romania joined the Allies in August 1916.

Brazil , China , Greece , United States of America joined the war in 1917.

Other countries such as Guatemala, Haiti, Russia, German Empire, Belgium, Britain, Honduras, Nicaragua, Panama, San Marino, and Siam were also involved in the Great War.