User blog comment:Argorrath/Treyarch releases a BO2 teaser video called "Origins" for DLC 4/@comment-1404296-20130808042510/@comment-1187559-20130808112654

Japan was in WW1, but they didn't fight on the Western front. In fact in WW1 America, Russia and Japan didn't really do that much, in comparison to WW2. America came in towards the end, and frankly were just extra troops, there were never any big operations. Russia was only involved for a short while, until the Communist revolution when they pulled out. And Japan was only attacking German colonies in Asia, but nothing to greatly aid the overall war effort, and nothing that we really remember when WW1 comes to mind.