How did people who were social Darwinist justify imperialism?

By the theory of natural selection.

Explanation:

Many proponents of imperialism have invoked Darwin's theses, that is, the natural selection of which Darwin speaks. In this context, he points out that it is natural for there to be a struggle for resources because these elements are recorded in the natural environment. In such circumstances, the stronger are superior and have a natural right to own these resources. Also, Darwin pointed out that the strongest are best adapted to the environment and conditions. In that way, many proponents of imperialism emphasized the mentioned norm to protect imperialist ideals.

yes

explanation:

why

it was the "4. potsdam" conference that demanded the unconditional surrender of japan, since it was clear that the japanese would continue to find until the very last man otherwise.