The left spits "Nationalism" like it's a bad thing. There's Tribalism, Nationalism, and Internationalism (globalism). I find nationalism the least offensive and most tolerant of the three: the former has no tolerance for people outside their tribe/clique, the latter has no room for people outside their ideology. But, "Hitler was a Nationalist". No, Hitler was a progressive, collectivist, statist, authoritarian, imperialist and socialist, but the head of the National Socialist Workers Party was not very nationalist at all.
Socialism is Nationalism at its core: National Socialism is almost redundant.
Socialism (collectivism), is the idea that you create a group (collective/insiders) and protect them from the market forces of an unjust world. And by nature of that, that means there are outsiders (those not part of the community) that must be protected against. So "us vs them" is a prerequisite of Socialism: sometimes it businesses, other times those that have private property, or those that won't share -- but there's always an enemy in Socialism (that must be re-educated, driven out, or ruled over). The philosophy of "the collective", means the exclusions of those "not in the collective" -- collectivism is about exclusion.
Usually, they build these boundaries on culture/country and ideology -- but it's easy to see how that devolves into race, nation, or just hating Trump supporters: contempt for anyone that doesn't belong. Socialists are NOT fighting for the betterment of humanity, just those in their mean-Girls club -- which explains how they're not for free trade, and are isolationist and protectionist, even if knowing those policies hurt the poor in other nations.
Socialists will fight against the 1%.... but point out those making more than ≈$30K are the 1% globally and ask them if they willing to tax our poor at 90% and give it to the rest of the world, and they look at you like you're a leper asking for oral sex. They don't want to distribute more to those outside their clique/club, are you nuts? So whether nationalism is built on national borders, racial ones, or just the borders of their belief, Socialists are almost always exclusionary nationalists.
Oversimplified Hitler's problem wasn't that he believed making Germany the best country he could, or that he thought it was the best place/culture. That jingoism is harmless and good for countries... to a point. Go Team America, Fuck Ya! Who cares if we cheer our team at the olympics, or take pride our accomplishments and strive to be better? That's not bad. If your national neighbors have 5x the murder rate and corruption, then you might want to filter which one of them you let in: for the same reasons you shouldn't let a Clinton or a Kennedy babysit your little girls.
Hitler's problems were that he was a progressive-collectivist-racist, who didn't believe in individuals over the state, and he excluded many citizens from being "true Germans". So that he could exclude 20th generation Jews as not being "true" Germans and treated them like sub-humans? Now that's a problem.
It's almost like when the far-left says all right wingers are Nazi's (and sees them everywhere), and then says/thinks, "punch a Nazi in the face" is a valid work view. Or when Black Lives Matters sees racism behind every badge (or in every white person who doesn't profess their privilege), and then thinks it's OK to be a bigot, vandal or assault people because of it. It's that blind, non-introspective hate, that scaled poorly in WWII Germany.
The other big problem is that Hitler wasn't a nationalist. If was a Nationalist, he wouldn't have wanted to leave his borders and invade. (He would be happy in his nation, with his people). The problem was that he believed in imperialism, and that he knew more than others, and could make the world a better place by telling others what to do. (Like all Socialists believe). So forcing his world view on others by force, that's a tenet required of Socialism -- but not of Nationalism.
Remember, if "that wasn't real Socialism" is the defense of every failed socialist country (like in the case of National Socialism), then that means, "that wasn't real Nationalism" is the obviously valid defense of National Socialism. Since America was probably most Nationalist before the progressives took over, and we went from being nationalist to being imperialists. Switzerland is one of the most nationalist countries in the world.
So that the Fascists (National Socialists) marched around singing nationalist (self-praising) songs, and showed their Hugo-boss pit-stains to each other? That was the least of their problems. The they were progressives, collectivists, Socialist, statists, totalitarians, authoritarians, or just plain so arrogant that they refused to look at the consequences of their own actions (like progressive leftists everywhere)? Now THAT was the problem.