Austria-Hungary : Austria-Hungary initiated World War I by declaring war on Serbia in July 1914.
Belgium : Though initially neutral, Belgium joined World War I to offer stiff resistance against German invasion.
Brazil : Brazil joined World War I in 1917 on the side of the Allies, and was the only Latin American country to participate in the Great War.
British Empire : Britain and British colonies including Australia, Canada, India, and New Zealand formed part of the Allied camp. The spread of the British colonies across the globe brought World War I to Africa and Asia.
Bulgaria : The Kingdom of Bulgaria joined the Central Powers in 1915 and fought until September 1918.
China : China entered World War I in 1917 following U.S. entry into the war.
France and Colonies : One of the major Allied nations, France declared war with the German declaration of war against France on August 3, 1914.
German Empire : Germany was a major Central Power. Austria-Hungary’s declaration of war was heavily influenced by Germany’s assurance of support.
Greece : Following the U.S. entry into World War I, Greece entered the war on the side of the Allies on June 27, 1917.
Italy : Initially reluctant to join the war despite an alliance with Germany and Austria-Hungary, Italy sided with the allies in 1915.
Liberia : Liberian trade was adversely affected by World War I, and the country joined the Allied Powers in 1917.
Montenegro : Linked closely with Serbia, Montenegro joined the Allied cause in August 1914.
Ottoman Empire : The Ottoman Empire joined World War I in November 1914, on the side of the Central Powers, due to its close ties with Germany.
Portugal : Despite the rivalry between Portugal and Germany, Portugal remained neutral until March 1915, when Germany declared war. Portugal then joined the Allies.
Romania : Romania joined the Allies in August 1916.
Russia : Russia, along with Britain and France, was one of the major Allied Powers, and first among the nations to mobilize troops against Germany.
Serbia : The outbreak of World War I was triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary in Sarajevo, Serbia. Austria-Hungary declared war on Serbia in July 1914.
United States of America: The United States declared neutrality in 1914. In 1917, the United States associated with the Allied nations thereby changing the course of World War I.
Other countries such as Guatemala, Haiti, Honduras, Nicaragua, Panama, San Marino, and Siam were also involved in the Great War.