Per Wiki
U.S. President Woodrow Wilson’s claimed objectives for sending troops to Siberia were as much diplomatic as they were military. One major reason was to rescue the 40,000 men of the Czechoslovak Legion, who were being held up by Bolshevik forces as they attempted to make their way along the Trans-Siberian Railroad to Vladivostok, and it was hoped, eventually to the Western Front. Another major reason was to protect the large quantities of military supplies and railroad rolling stock that the United States had sent to the Russian Far East in support of the Russian Empire’s war efforts on the Eastern Front of World War I. Equally stressed by Wilson was the need to “steady any efforts at self-government or self defense in which the Russians themselves may be willing to accept assistance.” At the time, Bolshevik forces in Siberia controlled only small pockets, and President Wilson wanted to make sure that neither Cossack marauders nor the Japanese military would take advantage of the unstable political environment along the strategic railroad line and in the resource-rich Siberian regions that straddled it.[1] Anticommunism was also a strong factor.
Concurrently and for similar reasons, about 5,000 American soldiers were sent to Arkhangelsk (Archangel), Russia by Wilson as part of the separate Polar Bear Expedition.
The Japs, who were our Allies in WWI, never forgave the US for that.