It's not that America is trying to be more diverse. Rather, America always has been diverse. In what's now the US, long before any white people settled here, there were already over 500 nations, many with overlapping territory or even no territory other than customary migration routes.

Then white people settled in the US, bringing slaves mostly from Africa. And people of every nation eventually came here. With the exception of the slaves and perhaps refugees who only migrate to flee bad conditions in their old country, people come here of their own free will.

This is what happens when you allow people to go and live wherever they want instead of forcing everyone to stay put. If they like where they were born and raised, they'll stay. If they don't, or even if they do, but find somewhere that's even better for them, they will move. Even if it means moving to live among people who look differently and have a different native language and culture.

And by the way, Japan's going to get more diverse, too.