If diversity is such a strength then why is it that the US and all other White countries reached their pinnacle of success when they were 90% White or more?

It's an inverted double scale; civilization declines as Diversity increases. That is what happened to Rome and in fact every empire that has ever existed, including the Soviet Union. The U.S. became an empire by principle once it made blacks citizens and then followed with other racial and ethnic groups. The Marxists knew this would happen, so they supported the pressures that brought about the Civil War. In fact, the NAACP was created by Marxists who were all white with one exception, the mulatto WEB DuBois who hated whites even more than himself. (He married a white woman. I believe marrying outside of one's race is rejection of your own racial/ethnic identity).