Multicultural societies have always existed (long before the US anyway). Ancient greeks were multicultural, the roman empire was multicultural and also, maybe the best example, the byzantine empire. These civilizations are the ones that historically have shown the most progress.
There's a process called synthesis, which no racist twat will ever understand. Synthesis of cultures, synthesis of ideas etc.
In fact, the idea of nation states is quite recent. Nationalism as a word for example was first used in 1770 by Johann Gottfried Herder. At the time of inception it was actually a very useful and revolutionary idea because it allowed people who until then were loyal to a city or a feudarch to gain common consciousness and fight a common battle for their rights (american and french revolution). So basically, at the time of inception, nationalism was a unifying term and not a dividing one.
Now, especially in the case of the american revolution it is actually quite funny because the 'nation' doesn't have any common ethnic cohesion. There was no common 'blood', only a common cause. Still, nationalism managed to UNITE all those people from different backgrounds under the same flag.
Today's nationalists are a very different story. The are populists who use the idea of the 'nation' for their own political interests, usually modifying it to their needs by excluding certain groups of people who they demonise in order to gather dumb fanatic follower behind a mad leader. They abuse the national idea to gain political influence and they do it in a way destructive to society. They use the weak minorities as targets, not for genuine reasons, but in order to attract politically those they can piss of with that group. Hitler for example would have never been elected if he didn't do exactly that: Have Goebbels demonize the jews and fags to get himself into office. Any leader who comes to power with this strategy is nothing but a mad fascist power hungry dictator that will fuck his nation up when in power to push forward his personal interests/madness.
Such leaders should be shot on sight and their followers should be institutionalized.
The US won WW2 by beating the nazis. Some nazis though changed their identity and managed to infiltrate the US. 60 years later they seem to have polluted US politics and they're winning the americans in an unfought battle.
Shame on their american followers who betray the ideals of their own country and convert to the globaly looked down upon ideology of the mos fucked up dictator in modern history.