... well, Japan and Germany didn't conduct much of common actions, that's true.
But I would assume, that Britain and France and the US - at least economically (even though probably underestimated in its abilities) - tied up to a great extend in fighting Germany, Britain to surrender in short thought of as a very possible option at that time, triggered Nippons attacks NOW for quite some extent.
On the other side :
without a mighty Germany going to war - against the Wallies as well as Stalin - there might have been other options for them to exlpoit
- go back on the SU again "joined" with the wallies against Bolshevism
- team up with SU against the Wallies (depending on how greedy Stalin might have been on far east aquisitions)
(well, I'm far from being an 'expert' of the japanes at that time, just my 2 cents)