China remains the hardest contradiction in an Anglo-Japanese alliance

Would Japan still invade China if it aligned with Britain against Germany and Italy?

The Sino-Japanese War was already underway, so the key uncertainty is whether Japan widens it and whether Britain continues supporting China after accepting Japanese help.

This counterfactual does not erase the fighting that began before the European war. Japanese deployments to the Mediterranean or Europe could limit the resources available for a larger offensive in China. British aid routes also ran through imperial territory and could be curtailed if Japan became a crucial ally. Churchill would face a severe strategic tradeoff between condemning Japanese conduct and accepting forces that materially improve Britain's survival prospects.