Western and Japanese forces were ostensibly sent to Vladivostok to rescue the Czech Legion. The legion was formed by the Russian Empire from captured Austrian soldiers of Czech descent who wished to fight against their German-speaking overlords. When the revolution happened the legion found themselves in a sticky situation.
"The Allies convinced the Bolsheviks to withdraw the Czech Legion from their crumbling front and ship them via train 6,000 miles across Russia to Vladivostok where they would be transported to the western front and continue the war there. Some 9300 kilometers (5470 miles) of this trip was across the Trans-Siberian Railway from Moscow to the Pacific Ocean.
By the time the Bolsheviks signed the peace treaty at Brest-Litovsk in March 1918, the Legion had been pulled from the line and was well on its way, strung out across the Trans-Siberian Railroad from Chelyabinsk in the Ural Mountains to Vladivostok on no less than 259 trains. The Germans demanded from the Bolsheviks that the Legion be disarmed and sent back to Austria along with the millions of other Central Powers’ prisoners of war that were being returned. When the weak Bolshevik Red Army attempted to detain and disarm the Czechs in May, the Legion acted in self defense and seized the railway. The force, which had swelled to some 50,000 armed men, was the best trained and equipped group in Siberia and they quickly seized most of the sparsely inhabited country along the railway. They captured Vladivostok, Chita, and Omsk in June and began moving towards Moscow.” -by Christopher Eger