In the New Earth Miniseries this came up, and so I did some quick research. It was very interesting.
With the Bolshevik Revolution in 1917 the Entente Powers (Britain and France) sent troops to Siberia in support of the White Russian army.
1918 – 1922
These troops went into Russia via the northern polar religions and eastern Siberia. The USA sent troops to both, and Japan sent 12,000 troops to Siberia. China sent 2,000 troops upon invitation of Chinese already in Russia. The Canadians and Italians also sent troops. The Czech’s had an entire legion there trapped.
While Japan sent 12,000 troops they had refused an earlier request from the French. The Japanese saw their own opportunity of making a separate buffer state of Siberia.
Siberia is predominately a Buddhist community and Asian, so there were ethnic grounds to make this happen.
The Japanese troops went along the Manchurian border where they met up with another 70,000 Japanese troops from Manchuria. When the allied withdrawal began in 1919 – 1920 the Japanese did not withdraw.
The USA placed intense diplomatic pressure on Japan to withdraw, and under high demand, the Prime Minister announced the withdrawal in October 1922.
While the allied purpose was to rescue a lost Czech Legion, and prevent western weapons falling into the hands of the Bolshevik Red Army, the Japanese were extremely anti communist, and either had territorial aims in the region, or simply wanted to set up a buffer state and end the northern problem.