This is not an area of expertise for me, others on the forum will know more but I will chime in with what I do know.
US forces went to Russia, essentially, in an effort to keep Russia in the war and influence the Bolshevik Revolution, though some historians have other interpretations. The Americans were only a small contingent among a much larger effort by the Western Allies and the Japanese. As I recall the given reason was to "safeguard supply depots" however. US troops were involved at Archangel and as I recall also near Vladivostok. I can't recall the units offhand but will try to find out. The first Allied troops began landing around March 1918.
As an aside, and you probably know this, US forces were equipped for the operations in Russia with American-made (Westinghouse or Remington) Mosin-Nagant M1891 rifles, an excellent idea given the supply issues involved.
Hopefully a more detailed reply will be posted by another but I will see what else Ihave on the matter.
To Join the Company of Military Historians click here