I'm going to violate Godwin's Law here, and compare Trump to Hitler to explain why Trump is so dangerous. Hitler correctly identified many, if not most, of the problems of the Wiemar Republic, but then laid the cause of those problems at the feet of groups of undesirables, the Jews, the Masons, the Gypsies, the homosexuals. By choosing groups that were already disliked, he looked like he was solving all of Germany's problems, making it "great again."
Trump is taking a page right out of the same playbook, identifying the problems correctly, but then laying fault for them on the "other:" Mexicans and Muslims in particular. This creates a culture of fear that is then exploited for political power because it is always easier to blame others that to find fault in yourself.
It gives the illusion that the problems facing the nation are easily solved through some sort of Final Solution, i.e. eliminating the undesirable class, whether through Gas Chambers, or just mass deportations and revocation of citizenship. This allows the problem to be solved without any fundamental change.
Adding to the parallel, the Wiemar Republic was one of the most Liberal societies in Europe, which caused a profound shift in cultural values, thereby upsetting a larger swath of the population than normal. This shift set the stage for a conservative demagogue, because the other root cause of the nation's problems seemed to be a libertine culture. By fighting the Other and the Culture, he could promise, much like Trump, to restore the prestige of Germany.
Finally, Hitler took advantage of the "apology" for World War 1, and the crushing reparations laid on Germany. By claiming Germany did no wrong, he instilled nationalistic fervor. Trump is playing the same card, but even more deceitfully, by claiming Obama is "apologizing" for America, rather than conducting diplomacy.
The United States made several terrible decisions on the world stage, and Obama has owned up to those mistakes. However, this has set the stage, again, for the Trump to claim that this causes the U.S. to loose prestige, rather than gain it. The idea, held by both Hitler and Trump, is that the United States needs to be feared, not loved, as that is a core pillar of Nationalism.
In all, this is a perfect shit storm for our country, and a dangerous precipice to walk.