What if Japan attacked America but not Britain?

What if Japan attacked the United States but did not attack Britain or invade British possessions?

It creates a serious coalition problem: Britain might lack an immediate casus belli, while a U.S.-Japan war without Germany's declaration could complicate American support for the European war.

The scenario asks whether Britain could have justified war if Japan had left Malaya, Singapore, and the British Empire untouched, and whether Washington could still supply Britain heavily while fighting only in the Pacific. It becomes especially consequential if Germany also avoids declaring war on the United States. Axis coordination and target selection linked the Pacific and European wars.