You'd need a lot of conditions for this one...
First and foremost, Japan would need to come out of World War II undefeated.
Germany would need to be contained. Though not crushed to the point where the Allies and Soviets are stealing their Rocket scientists as per OTL.
Keeping the US in isolationalism wouldn't hurt.
So you'd need a war that would cripple basicly every European power somehow and leave Japan on top somehow.
Taft getting elected in 1940 would keep the US in effective isolation. That could very easily stall the European war without a victor, while Japan could sieze some chunks of the far east at her leisure, especially if Taft doesn't turn off the oil.
Perhaps a nuclear exchange between Britain in Germany in the late 40's, crippling Russia, Britain and Germany forcing some form of peace, while Japan gets to walk away with it's Pacific empire?