Quote Originally Posted by Franconicus
July 1918 Japan and the US send troops to Siberia to assist Czech troops trying to get to the Western front. While US troops only grow to 7,000 men, Japan eventually sent 72,000. (do not understand this!)
This was part of the general Allied intervention in the Russian Civil War. There were two expeditions, one from Archangel south towards Moscow that actually fought Bolshevik forces for a short time and a second from Vladivostok west. The latter one was the force supported by the Japanese. It's objective was to assist the Czech Legion. The Czech Legion was a large division of Czechs and Slovaks who had deserted from the Austro-Hungarian army or volunteered after being captured by the Russians. They served as part of the Russian army on the eastern front until Russia surrendered.

At that point, the Bolsheviks gave them free passage east to the Pacific via the Trans-Siberian railroad. The eventual intention being that they would then be shipped to France to continue the fight on the Western Front. Due to the general chaos inherent in the Russian Civil War, things didn't go as planned. The Czech Legion eventually found themselves unable to continue to the Pacific and wound up fighting with the Whites. The US expedition was originally sent to help rescue this group of stranded Czechs.