hm, for some reason the pacific is nearly always an important part in our games.
if jap doesnt have the biggest fleet, it would lose a lot of income to the US due to losing all those islands. and if the US doesnt hold against that, jap will sooner or later land in los angeles.
furthermore, on the main continent, jap is usually under pressure from uk (india) and the us (china). so its actually not too unhappy about the NAP with the russians.
all this, while its not really about killing jap first… but those situations seem to arise in a chain reaction: the US has to help the UK, by going to africa and by its presence in the pacific, which forces jap to invest in fleet as well… instead of building troops to invade india/china.
due to that, the UK can help russia by nagging the germans.
the better the allied nations support each other like that, the less likely it is, that germany or japan manage to make any big moves.