Other factors to consider in the "states versus federal" conflicts that could occur are that each state has its own National Guard forces and equipment which are under the state governor's control. The National Guard are under dual control in that they can respond to the state's needs or to federal needs. But they are still citizens of that state who put on the uniform when needed.
This could lead to National Guard versus federal forces stand-offs as was seen in the 1960s over Civil Rights disagreements between state and federal governments:
https://en.wikipedia.org/wiki/Little_Rock_Nine#National_Guar...
Another factor that differentiates the United States in conflicts of the people against their government is how heavily armed and resourceful the US populace is. In the War on Terror, US Armed Forces faced insurgency militias in Iraq and Afghanistan. If similar insurgency militias were to arise in the United States in response to illegal federal government actions, it would probably have similar results.
https://en.wikipedia.org/wiki/Insurrection_Act_of_1807
Is not optimistic reading
Indeed. That law and several others grant the president expanded powers to handle emergency situations. However, who determines what is an emergency and if the expanded powers are needed? In many cases, it is the president himself (or herself).
The underlying assumption is that the president would use such powers judiciously and that the expanded powers would enable emergency situations to be handled quickly since it would probably take more time for congress to respond and get things done.
The question now is: what if such powers are not used judiciously? What recourse is there?
> who determines what is an emergency and if the expanded powers are needed?
Congress. But so far they're letting Trump declare emergencies left and right. All of his tariffs are being enacted under an "emergency" to bypass Congress (since under non-emergency situations, Congress sets tariffs).
Had the Democrats won control of the House/Senate, a lot of this nonsense wouldn't be happening (or even if conventional Republicans had control, which is why Trump 1.0, when conventional GOP held the senate, Trump couldn't go off the rails as he has now).
There's also State Guard / State Defense Forces, which are solely under state jurisdiction. But in many states, and especially in blue states, it has devolved into a uniformed and militarized but unarmed organization.
However, this entire line of thought presupposes that those people (whether in NG or SDF) would align themselves with the state and against the feds, and that's not given at all. I know from personal experience of close interaction with my local right-wing militias in WA state that quite a few members are in NG or WSG.
The same goes for armed populace. It's true that there's a lot of weaponry in private hands, and we're not just talking your stereotypical AR-15, but stuff like say .50 BMG anti-materiel rifles, grenade launchers, and even privately owned tanks and artillery in some cases. However, they are disproportionally in hands of people who lean far right, so in event of open conflict I would expect them to work with the feds. There are some of us on the left who are heavily armed precisely so as to counterbalance that, but we are outnumbered by an order of magnitude. Then there's the issue of training - right-wing militias actually get together and train, and while it is derided as LARPing - often for good reasons - it's still better than nothing. More importantly, it's not just training but also networking - those people know each other and have plans to get together and coordinate "when it's time".
Indeed, a particularly nasty possibility is that Trump wouldn't even need to issue any kinds of explicit orders to federal troops, but rather just let the right wing paramilitaries loose by simply not doing anything to stop them (and making it widely known that there will be no consequences).