Original: 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.
Simplified: 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.