How is loosing territories some 5.000-10.000 km away an "existential threat"? Even if they wanted to, Japan had no means of successfully invading main land US.

The US justifies dropping the Nukes with it preventing an extraordinary loss of life if they had to stage an amphibious invasion of main land Japan. But at least the US could stage much closer to Japan, than Japan could to the US.

In the same wake the Britains loosing their empire was not an existential threat to the US just as much as the genocide against China was not an existential threat for the US just as the Holocaust and the genocides in eastern Europe weren't an existential threat to the US.