This answer I wrote might have some useful background on the US intervention.

The United States sent troops in two operations: about 5,000 troops in the 339th Infantry Regiment were sent to Archangelsk in September 1918, where they were part of a larger force of mostly British, Canadian and French forces, which eventually were allied with local White Russian forces. The initial remit of this Allied expedition was to guard war materials in Murmansk and Archangelsk, and deter possible German aggression to these ports (for context, this was around the time that Germany had dispatched troops to Finland). However, the North Russia force very soon got involved in driving local Bolshevik forces out of the towns in question, and pursuing them further up the Northern Dvina River, setting up a very wide screen in the countryside. This was all during the First World War...and these actions began to be seen as rather pointless from November 1918 on, especially as locals in the area really didn't show much enthusiasm for getting involved into the rapidly-escalating Russian Civil War. With the arrival of British Major General William Edmund Ironside, taking overall command of the expedition in late November 1918, the forces shifted to a defensive posture and were much better led and organized...but initiative mostly shifted to the Bolsheviks (who were able to infiltrate defensive lines on skis and sleds), and who mounted a series of attacks in January 1919, forcing the forces to begin withdrawing to Archangelsk. The Americans began withdrawal preparations in April, and were officially out by August. The British forces remained until that fall, and the towns eventually fell to the Bolsheviks in February 1920.

The second US expedition was to the Russian Far East. US troops were dispatched in August 1918 (about 8,000 total would be stationed there), and were finally withdrawn in April 1920. They were theoretically under the overall command of a much larger Japanese occupation force (which would at its highest reach 72,000) that first landed in Vladivostok in April 1918 and did not finally withdraw until 1922. These troops were under the command of Major General William S. Graves, who in actuality operated somewhat at odds with the Japanese forces. His instructions as received from Secretary of War Newton Baker in Kansas City in August 1918 were as follows:

• Facilitate the exit of the Czechoslovak Legion from Russia. The legion had seized control of much of the Trans-Siberian Railway and so a big part of this was for the US forces to operate the railway in the Far East.

• Guard US war material stockpiled in Vladivostok.

• Help establish local government in the Far East. This was...a very vague directive, and one that more or less put US forces into an open-ended peacekeeping position. It wasn't explicitly *anti-*Bolshevik (there were some elements in the US government that considered the Bolsheviks the democratic choice), but it wasn't really *pro-*Bolshevik either. The major concern was arguably to prevent the Japanese forces from building up major warlord proxies in the region - which is pretty much what ended up happening, including such notable (and notably destructive) figures as Grigory Semenov (Graves said of Semenov that he was the "worst scoundrel I ever saw or heard of"). Local Bolsheviks initially avoided conflict with the Americans, but a series of sharp, violent battles between the two forces broke out in the summer of 1919, as the US forces attempted to maintain control of railways to coal mines in Novitskaya for use on the Trans-Siberian Railway. Some 30 Americans were killed and 50 wounded (a further 139 died from disease or accidents in the campaign). Ultimately, the purpose of this expedition became increasingly unclear (it wasn't clear who was a "legitimate" local Russian authority, the war material the troops were supposed to guard was for a war that had already ended and that were rapidly dwindling from theft, and the Japanese seemed to have much different designs on the region than the Americans) and it was ultimately withdrawn.

So in answer to the original question: Why were American troops in Russia? They were initially there for the war effort in the First World War, notably to guard caches of war materiel in Murmansk, Archangelsk and Vladivostok. They were part of much larger Entente commands who had landed in the spring of 1918 for this purpose, and in Siberia at least there were added objectives of helping the Czechoslovak Legion extricate themselves (again, so that they could ultimately be redeployed against the Central Powers), keep the Trans-Siberian Railway open, and try to counter more aggressive plans by the Japanese occupation forces as a counterweight. The Northern Russia expedition was much more actively involved in fighting the Bolsheviks, but much of this was before November 1918, when it seemed that this was part of the First World War effort. Once the Armistice came, it became much less clear just what the expedition was doing - they certainly preferred local White forces. But even these forces (the Northern Army organized under Yevgeny Miller) were largely captured Bolshevik forces pressed into service (who would ultimately flip back and give control of Archangelsk and Murmansk to the Bolsheviks), and much of the activities of this Army occurred in mid-1919, when the American troops were mostly on their way out.

One final historiographical note is that Soviet perceptions of the US intervention changed over time, and were very much connected to the state of diplomatic relations between the two countries. In the 1930s, when formal diplomatic relations were established between the US and USSR, the Soviet government emphasized that US forces in Russia during the Civil War were there for positive (if misguided) purposes as peacekeepers. That rapidly changed with the onset of the Cold War, and by the 1950s the Soviets were framing the American intervention as imperialist attacks on Russia and on the communist movement, and just the beginning of a longer conflict (the Siberian Expedition began to be tied to American "aggression" in Korea, for example). All in all, the US forces involved were very small, even by Allied Intervention standards, and really were a small sideshow to the First World War: even at the time military and political leaders weren't really sure what those troops were supposed to be doing, or even where they were exactly.

Some sources:

From my original post, these are a couple good books that touch on US and Entente intervention in Russia:

Adam Tooze. The Deluge: The Great War and the Remaking of Global Order 1916-1931

Stephen Kotkin. Stalin: Paradoxes of Power, 1878-1928

Alexander F. Barnes and Cassandra Rhodes have two articles on the expeditions in Army Sustainment, which is the US Army's logistics journal (professionals study logistics!):

"Logistics in Reverse: The U.S. Intervention in Siberia, 1918–1920". Army Sustainment, Volume 44, Issue 1 (Jan-Feb 2012), available here,

"The Polar Bear Expedition: The U.S. Intervention in Northern Russia, 1918–1919." Army Sustainment, Volume 44, Issue 2 (Mar-Apr 2012), available here.

Otherwise a lot of literature on the interventions is pretty old at this point, like George F. Kennan's Decision to Intervene: Soviet-American Relations, 1917-1920, or Betty Miller Unterberger's America's Siberian Expedition, 1918-1920: A Study of National Policy. There are a few popular histories out there, and some newer works like John M. House and Daniel Curzon's The Russian Expeditions, 1917-1920 or Robert L. Willett's Russian Sideshow: America's Undeclared War, 1918-1920