President Wilson agreed in July 1918 even though the War Department said not to help . The U.S. sent two groups of soldiers : 5,000 soldiers were the American North Russia Expeditionary Force ( nicknamed the Polar Bear Expedition ) ; and 10,000 were called the American Expeditionary Force Siberia . In July 1918 , against the advice of the War Department , President Wilson agreed to send 5,000 U.S. troops as the American North Russia Expeditionary Force ( aka the Polar Bear Expedition ) and 10,000 U.S. troops as the American Expeditionary Force Siberia .