> Wasn't always the case...

Sure, but it has been for the better part of a few decades. The whole reason US hegemony has spread so far and wide is due to this.

> Genuinely interested! What are those safeguards and what do they prevent that happened with the Weimar republic?

I'm not American so I'm probably getting the tiny details wrong here so please correct me if I'm wrong on any points. A lot of this is going off my memory, so I'm probably getting some dates and such details wrong as well. I'm definitely not including a very comprehensive answer here, as it's a complex topic with a lot of history attached that I don't know too much myself. I'm mostly just a nerd who finds this kinda stuff fascinating, not any kind of expert :)

The big sticking points for the Weimar were that the president wielded much more legislative and executive power than US presidents do. Article 48 let the Reichspresident call a state of emergency without ever involving the Reichstag (Parliament) which basically enabled them to become dictators whenever they wanted. Article 48 was one of the early keys Hitler used to seize power, as a fire in the Reichstag parliament house gave him an excuse to call a state of emergency because of a supposed Communist uprising. He used Article 48 to arrest Communists en-masse on the basis of the Reichstag Fire Decree which was signed shortly after the fire, which also included many provisions that restricted free speech, movement and other similar civil liberties. I'd recommend further reading up on the Fire Decree yourself, as it's quite interesting as a key turning point in the Weimar turning into Nazi Germany.

In contrast, US presidents cannot supersede congress and decrees are subject to congressional oversight (there probably exist exceptions, so take my words here with a grain of salt). Even emergency powers (such as the ones Hitler used) are much weaker for US presidents and have to go through congressional approval. Even if every single member of congress is a republican, republicans are not a completely united party. A lot of them dislike Trump and have their own agendas they'd prefer to be pushed, and ultimately they have no real reason to bow to the president since they are elected in completely different timeframes, wield different but almost equal power and are also competing with every other member of congress. For example the fear mongering about leaving NATO, there's basically a 0% chance of that happening because it requires a supermajority from congress, despite whatever the President might want. It's a pretty common reason why things like the recently proposed student loan debt forgiveness never end up happening, the president can't just will it to happen.

Another big one is that the militaries work under different philosophies and circumstances between the two, and you can't have a takeover without military backing. The Weimar military was still pretty loyal to the old monarchists and viewed Weimar as a forced state that they were put into under pressure after losing WW1. You have to understand that the whole "democracy" idea was a pretty fresh one at that time for Germany, they only switched from monarchism to republicanism in 1918 after the November revolution.

By contrast, US military as far as I understand it isn't really all that loyal to whoever the current president is, but rather to the constitution. The president might be commander-in-chief, but that doesn't mean he can tell the military to do whatever they want. They still wield power over the military of course, but it's a lot less pronounced than it was in Germany, because the military were loyal to Hitler. If the military leaders who are ultimately the ones commanding the troops don't like the president, there isn't much they can do. Even the national guard is interesting, since it's a split responsibility between states and the federal government. And, again, congress also has a say in many military things, though my knowledge there is for sure lacking so I'd recommend you do your own reading up there.

An example there of the limited power of the president was when Nixon was getting the boot, the secretary of defence James Schlesinger at the time instructed military leaders to run Nixon's order by either him or the secretary of state, because he was worried about Nixon's reaction.

And again, the economic and social situation in Germany at the time cannot be overstated. People were miserable, the country was massively poor and were in a major demographic problem due to the war. Their industry was quickly stagnating due to the aftermath of WW1 and there was a lot of resentment building up in Germany for what they considered to be unfair and harsh treatment from the Allies. They were, to put it charitably, extremely unstable times and it was a matter of time before all of it exploded like it did. If it wasn't Hitler, it would've been the next charismatic leader promising to take revenge on the people who ruined the country (which is massively oversimplifying things of course, but you get the gist)