In World War I, the main conflict was between two major coalitions:

• The Allies (Entente Powers): Primarily France, Russia, the British Empire, and later Italy (from 1915), the United States (from 1917), Japan, and others. These countries fought together against the Central Powers

• The Central Powers: Mainly Germany, Austria-Hungary, the Ottoman Empire (Turkey), and Bulgaria. They opposed the Allies throughout the war

The war began after Austria-Hungary declared war on Serbia following the assassination of Archduke Franz Ferdinand. This triggered alliances to activate: Russia supported Serbia; Germany supported Austria-Hungary and declared war on Russia and France; Britain entered the war after Germany invaded Belgium

. Thus, the countries fought primarily against the Central Powers coalition of Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria during World War I