The primary reason was the signing of a peace treaty by the new Bolshevik regime with the German government, which would lead to the collapse of the Russian front, and the isolation of the 50 000 allied troops stationed there. This would also allow the German army to move its divisions stationed on the eastern front to the western front, possibly re-enforcing its front lines before American troops arrived.
To celebrate the intervention the Tokyo based shobido and company produced a series of patriotic lithographic prints depicting various scenes from the campaign entitled The Illustration of the Siberian War 1919.
6) The brilliant exploit of the Noshido Infantry Company, destroying a rail road, going around the back of the enemy 1919.
7) The Japanese cavalry having taken possession of Khobarovsk, pursuing and attacking the enemy 1919
14) Our cavalry occupied Khabarovsk, and march past took place in front of the enemy's gun-boats 1919.
17) Our army attacks from sky, water and shore, and repulsed the enemy of Siberia 1919.
All The prints depicted here are courtesy of The Library of Congress Washington D.C.