When did each countries join World War 1?

World War I began on July 28, 1914, when Austria-Hungary declared war on Serbia. Russia mobilized in defense of Serbia on July 30, 1914, and Germany declared war on Russia on August 1. Following this, Germany declared war on France on August 3 and invaded Belgium on August 4, prompting Britain to declare war on Germany the same day. Italy joined the war later, switching sides to the Allies in 1915, while the United States entered the conflict in April 1917.