Just wondering, why is the US not moving before round 5? If germany attacks all sides all attacks are kinda weak. So they will lose a lot of power as well, They cannot attack france, and Russia and the UK fleet all in turn 1 and have a lot of forces left.
Also attacking russia round 1 isnt ideal and leaves some weak points.
Japan 1 is powerfull but it generally leaves some weak poins, not many but he cant expect to roll great everywhere.
As allies try to hit where you got some advantage, japan is weak if it loses to many inf units in china, so use that to drain him. Also dont leave forces open for easy conquest if he can attack with a lot of airpower then just make sure its only airpower or step back a bit and counter his small infantry stacks.
UK will have a massive fleet left, or germany has no airforce left, either way both UK and US can move into europe relatively quickly.