U.S has had the benefit of favorable geopolitics(a good read:http://www.stratfor.com/analysis/geopolitics-united-states-p...). A large living space and a fairly sturdy inland waterways.Historically, the pressure on natural resources has been mitigated by easier transport of goods and people(unlike India which doesn't have inland waterways). In last few decades U.S has also closed gates on immigrants for good. Though that has slowed down the process of ghettoization, eventually large swathes of U.S will convert into American-Hispanic and other similar sub-cultures. And then it's just "my tribe vs. your tribe". The U.S is at the beginning of the road, and in time when there is a greater pressure on resources, you are looking at yet another India within the next 100-150 years.

Let's just hope that it doesn't happen too soon and that human social mechanics today is advanced enough to handle another great civilization falling into chaos due to clash of cultures. The good news is that this time we have the Internet.

The US has certainly not closed its gates on immigrants for good. Immigrants as a percentage of population is close to the highs of ~15% seen in the late 1800s and early 1900s and has been steadily rising since the 1970s.

When my parents grew up in postwar NYC, they were part of a fairly insular Irish-catholic community. Sundays at church, weekdays at the church school. The church you went to was dictated by your country of origin. They talked to the Italians, but eyebrows were raised if you were hanging out with the Polish or god-forbid, some protestant.

Prosperity broke down the ethnic neighborhoods. This still happens today. Talk to Chinese kids raised in America what the family dynamics are like when you are an "ABC". Ditto for children of Indian descent or really any group with strong ethnic identity.

You see continued strong ethnic clustering where the ethnic identity is also associated with business and prosperity. Greek Orthodox communities are good examples of this.