I think that the heart of the problem lies in our allowing the philosophical proposition that there is no such thing as objectivity and therefore no such thing as objective truth, objective good, or (especially) objectively righteous standards* to be cast as a practical rule that there are no legitimate standards.
Doubting or discarding the norms which at least nominally or aspirationally defined behavior in America** led to the pernicious conclusion that any mode of life or culture is as reasonable, good, and valuable as any other. Thus we lost the will to demand that our own people behave in accordance with the standards that made America great, inclining our heads and smiling at wanton violence, braggadocio, preening, bastardy, dishonesty, and irresponsibility as aspects of a different but perfectly acceptable culture. We likewise lost the will to demand that immigrants assimilate, ignoring the fact that their cultures are what made their native countries cesspits to be fled rather than refuges to which to flee.
William F. Buckley once wrote that as Christianity is the universalization of Judaism, Americanism is the universalization of Anglo-Saxon culture. In other words, in the US anyone can become an Anglo-Saxon. At least in the public sphere, everyone should have been made to become one. What is the proof of this? America. And Guatemala, Mexico, Chad, and Myanmar.
*I personally believe in objective truth, objective reality, objective good, et c. What is up for debate is our capacity to divine or apprehend those things.
**Hypocrisy is the tribute vice pays to virtue.