If the US (and by the same logic) Japan were not involved in the war then Singapore would not have fallen we would not have had such a concern for resources being expended in Burma and the region. Drawing just on Commonwealth and Allied nation troops a second front could have still been opened, the war in Africa had been largely won and the Italian campaign could still feasibly have occurred.

However as has been said the war would have likely lasted longer but Russia would have been likely to have pushed right up to the French border, if not further and the geographical and political map could be very different today.

If Japan had still worked on their territorial gains to the West then who knows…