American nationalism is in fact the assimilation of many cultures: people who came here to be Americans, not create their own little nation within a nation or syphon from public programs. I, like you perhaps, have friends who live in Europe. Many of them say they live in fear because the “open border policies” of their government which after decades of implementation threatens to destroy all that they considered the spirit and culture that was quintessentially “French”, “British”, “Italian” or even “German”.
It is a broad statement to assert that America, if it does not adopt the political philosophy of the left (Democrats), will be a nation of “white men”. This is just a ridiculous ploy by those who want to perpetuate a divided America. You speak about the murders perpetrated in the Nazi regime, but I would assert that our current leftist political climate, where disabled babies are murdered before having a chance at life, where others are murdered for the convenience of the mother, where otherwise “normal” babies can now be murdered after birth at the whim of a mother (who may be suffering from a from of depression), is no better, and in fact, is worse.
Nationalism does not require ethnic cleansing. It does require, however, that those who choose to immigrate here, move here for what America offers — which used to be a better life based on pursuing happiness and individualism. This does not seem to be the reality today, when the open border policy brings those who take what America has to offer without an effort on their part.
For a better perspective, I hope you will check this out: https://lifeintervening.blogspot.com/2016/06/whatever-happened-to-flag-waving.html