That is backward. In fact, US became much greater when European diversity came. Before that, it was largely a rural country with a small economy. The strength of US was the liberty it upheld, which allowed people from different background to strive with the best of their ability. Then people work together to better their lives. They naturally downplayed their differences or at least try to push them aside. After all, a dollar from a black is as good as a dollar from a white. That is why US is called melting pot. It doesn't focus on racial/cultural differences, like multi-culturalism does. It melts everyone into the same mentality, so called American dream.
Of course, there are many American politicians that do the opposite, especially those Democrats. They want to play the racial and cultural cards to trick people to give them power. They are the champion of institutionalized racism both in the time of slavery and now.