There are quite a few corners of the world where the ratio of immigrants to native-born citizens is remarkably high. Many of these places are small or rapidly growing countries - say, Monaco or Qatar. Some others, including several European states, just happen to be on the receiving end of transient, regional demographic shifts; for example, in the past decade, over 500,000 people moved from Poland to the UK. But on the list of foreigner-friendly destinations, the US deserves a special spot: it is an enduring home to by far the largest, most diverse, and quite possibly best-assimilated migrant population in the world.
The inner workings of the American immigration system are a fascinating mess - a tangle of complex regulation, of multiple overlapping bureaucracies, and of quite a few unique social norms. The bureaucratic machine itself is ruthlessly efficient, issuing several million non-tourist visas and processing over 700,000 naturalization applications every year. But the system is also marred by puzzling dysfunction: for example, it allows highly skilled foreign students to attend US universities, sometimes granting them scholarships - only to show many of them the door the day they graduate. It runs a restrictive H-1B visa program that ties foreign workers to their petitioning employers, preventing them from seeking better wages - thus artificially depressing the salaries of some citizen and permanent resident employees who now have to compete with H-1B captives. It also neglects the countless illegal immigrants who, with the tacit approval of legislators and business owners, prop up many facets of the economy - but are denied the ability to join the society even after decades of staying out of trouble and doing honest work.
Despite being fairly picky about the people it admits into its borders, in many ways, the United States is still an exceptionally welcoming country: very few other developed nations unconditionally bestow citizenship onto all children born on their soil, run immigration lotteries, or allow newly-naturalized citizens to invite their parents, siblings, and adult children over, no questions asked. At the same time, the US immigration system has a shameful history of giving credence to populist fears about alien cultures - and of implementing exclusionary policies that, at one time or another, targeted anyone from the Irish, to Poles, to Arabs, to people from many parts of Asia or Africa. Some pundits still find this sort of scaremongering fashionable, now seeing Mexico as the new threat to the national identity and to the American way of life. The claim made very little sense 15 years ago - and makes even less of it today, as the migration from the region has dropped precipitously and has been eclipsed by the inflow from other parts of the world.
The contradictions, the dysfunction, and the occasional prejudice aside, what always struck me about the United States is that immigration is simply a part of the nation's identity; the principle of welcoming people from all over the world and giving them a fair chance is an axiom that is seldom questioned in any serious way. When surveyed, around 80% Americans can identify their own foreign ancestry - and they often do this with enthusiasm and pride. Europe is very different, with national identity being a more binary affair; I always felt that over there, accepting foreigners is seen as a humanitarian duty, not an act of nation-building - and that this attitude makes it harder for the newcomers to truly integrate into the society.
In the US, as a consequence of treating contemporary immigrants as equals, many newcomers face a strong social pressure to make it on their own, to accept American values, and to adopt the American way of life; it is a powerful, implicit social contract that very few dare to willingly renege on. In contrast to this, post-war Europe approaches the matter differently, seeing greater moral value in letting the immigrants preserve their cultural identity and customs, with the state stepping in to help them jumpstart their new lives through a variety of education programs and financial benefits. It is a noble concept, although I'm not sure if the compassionate European approach always worked better than the more ruthless and pragmatic American method: in France and in the United Kingdom, massive migrant populations have been condemned to a life of exclusion and hopelessness, giving rise to social unrest and - in response - to powerful anti-immigrant sentiments and policies. I think this hasn't happened to nearly the same extent in the US, perhaps simply because the social contract is structured in a different way - but then, I know eminently reasonable folks who would disagree.
As for my own country of origin, it occupies an interesting spot. Historically a cosmopolitan nation, Poland has lost much of its foreign population and ethnic minorities to the horrors of World War II and to the policies implemented within the Soviet Bloc - eventually becoming one of the most culturally and ethnically homogeneous nations on the continent. Today, migrants comprise less than 1% of its populace, and most of them come from the neighboring, culturally similar Slavic states. Various flavors of xenophobia run deep in the society, playing right into the recent pan-European anti-immigration sentiments. As I'm writing this, Poland is fighting the European Commission tooth and nail not to take three thousand asylum seekers from Syria; many politicians and pundits want to first make sure that all the refugees are of Christian faith. For many Poles, reasonable concerns over non-assimilation and extremism blend with a wholesale distrust of foreign cultures.