I listened to a podcast interviewing Mark Koyama, economics professor at George Mason. He has a new book out, The Long Road to Religious Freedom. He sounded interesting and balanced, I would have hopes for the book. He draws a distinction between toleration and freedom, especially religious. Under toleration you are allowed to live, practice, and worship, but you have imposed limitations - offices or professions that are not open to you, extra fees or taxes you must pay. In freedom all people are treated equally.
The host asked why the idea of toleration, and then freedom, which turns out to confer a considerable economic advantage, took so long to take hold in society. Koyama gave two good answers. First, it is not obvious in the short run that it provides an economic advantage, and people prefer to give preference to "their own" in all senses of the word. Secondly, monarchs and leaders in general prefer to have as much unity in their subjects as possible, because a fragmented people is more vulnerable to attack from without. This is why secessionist movements are generally opposed, even when people don't like each other much. They hate the possibly invading Elbonians even more. The accusation is that rulers like to have power over as many people as possible, and while that is true it is not the whole truth. The people themselves like to feel they have many allies they can count on.
Related to this latter, a nation fears that those among them who are different might become a fifth column, or exert influence on foreign policy. Koyama gave as an example that Catholics in France and in America in the 17th-18th C's were very much interested in what happened to Catholics in Spain and Italy. Rather than brush this fear aside, as I fear many of the highly miseducated and even historians and social scientists would these days, he acknowledges that it is quite reasonable. History shows that this is quite true.
Americans are less likely to see it because the effect is weaker here. 1) We have people from many places, often specifically escaping the conflicts elsewhere and wanting nothing to do with them. The counter, that some still do care what is happening back in the old country, as in Ireland throughout the 20th C, Sudan, Latin America, the Middle East, is certainly true, but few of them care anywhere near as much as those still in place. 2) The various national, religious, and ethnic groups are somewhat offsetting, 3) there remains a powerful assimilative factor, however much it is under attack, that America is founded on ideas, not blood-and-soil. Because of that, most of any group that arrives here adopts America as its primary loyalty. Some do not, and that does raise questions as to how much of that "some" can be tolerated. In this sense, I don't mean "tolerated" in the sense of being nice people and believing that others will be nice as well, but in the sense of how much can be present before it puts all of us in danger, even if the rest of us are trying very hard.
In other places they have decided that not very much "some" can be tolerated, though Europe is having another go at it. They finally achieved peace after WWII as nations divided more along linguistic and ethnic lines than ever before.* Now they will bring in people even less like them than Poles are to Germans or Dutch to Frisians and see how that works. Americans have been doing this for four centuries, and if we aren't really all that good at it, we are at least ahead of everyone else.
*Europeans believe that they became a superior moral force because of what they had learned from devastating wars. Hmm, maybe. Ethnic populations moving back, and drawing the boundaries more cleanly around tribes strikes me as a simpler and cleaner explanation than a great change in human nature.