Russia's timeline in World War II is confusing enough, but its role in World War I was even more so. While most European countries were fighting each other, Russia had their own civil war, which lasted years and split the country into partisans, each with their own army: the Bolsheviks had the Red Army and the loose coalition of partisans opposing them had the White Army. And as the Great War ended, American troops were still in Russia.
When [President Woodrow] Wilson sent the troops to Russia in July 1918, World War I still looked dire for the Allies. With the Russian Empire no longer engaged in the continental struggle, Germany had moved dozens of divisions to France to try to strike a final blow and end the war, and the spring 1918 German offensive had advanced to within artillery range of Paris.
Desperate to reopen an Eastern Front, Britain and France pressured Wilson to send troops to join Allied expeditions in northern Russia and far eastern Russia, and in July 1918, Wilson agreed to send 13,000 troops. The Allied Powers hoped that the White Russians might rejoin the war if they defeated the Reds.
To justify the small intervention, Wilson issued a carefully worded, diplomatically vague memo. First, the U.S. troops would guard giant Allied arms caches sent to Archangel and Vladivostok before Russia had left the war. Second, they would support the 70,000-man Czechoslovak Legion, former prisoners of war who had joined the Allied cause and were fighting the Bolsheviks in Siberia. Third, though the memo said the U.S. would avoid “intervention in [Russia’s] internal affairs,” it also said the U.S. troops would aid Russians with their own “self-government or self-defense.” That was diplomacy-speak for aiding the White Russians in the civil war.
The US troops charged with fighting the Red Army in 1919 didn't understand why they were there, and neither did US politicians back home. American units were stationed at Archangel, in the Arctic Circle, and in the far east of Siberia, where the Japanese were making advances in the wake of Russia's upheaval. Read about the American involvement in the Russian Revolution at Smithsonian.