I just wanted to chime in here to say that the melting pot image of the United States was once a very true thing. From the 1700s to 19xxs (im honestly not sure when the EU started to comingle so much) but from that time period, the US was a country of people who immigrated from every corner of Europe, and even a large amount from Asia/South America. The rest of the world was much more confined to their own countries, and they were much more nationalistic. Germany and France in the 1900s...you would be hard pressed to find a significant amount of foreigners. In America however, cities were divided up into quadrants, its still somewhat accurate today where you have large concentrations of Chinese, Jews, Italians, Germans and so on in a specific part of a city. This cultural divide was happening WAY before Europe started to blend, which is why the USA has always spoken about how its a melting pot. Now, besides most of Africa and Asia, the entire world is a melting pot and people are moving more freely between borders than ever before (although ironically it doesnt seem like it because of the news). America was just way ahead of every other country for the most part when it came to this, now all major European countries are just as mixed if not more than the USA.
Also a side note- When the melting pot image of the United States existed between 1700-WW2ish, people saw Germans, much different from the British that came for instance. The world was much more religiously segregated, and people didnt speak the same language. Now all the white people you see in America (well 95%) have been here for a hundred years or more, and have no ties to their native language, which used to be very different than english, are predominantly less religious, and likely are a mix of every European countries' background. Many people here are part Irish, German, Italian, and English...just a complete mutt of traits from Europe. So people that once would've been considered different have just blended together much more so than they were before through language/physical traits/religion/culture etc. Hell i even have multiple friends who celebrate Christmas and Hannukah and are atheists. The USA is just a giant blender, but that doesnt mean everyone mixes nicely together