What would nationwide (all states) martial law be like if enforced in the US?

It would look like what Trump tried to do in Portland and Chicago.

It would be even less effective in rural areas where people are spread out and there aren't enough thugs to round everyone up into a small area.

i would love to see maga defend their own autonomy taken away.

It would be a massive shit show and would collapse rather quickly. Aside from how we've seen them 'contain' places like Portland, showing they don't have the skills they'd need to do it on a small scale, they literally don't have the manpower. From The Military Journal of Operations:

In the Winter 1995 edition of Parameters, James Quinlivan wrote an article (‘Force Requirements in Stability Operations’) arguing that force ratios are the best method for determining the appropriate force requirement. Quinlivan determined that a force ratio of 20 troops per 1,000 of the population is required for successful stabilization, which he defined as “[creating] an environment orderly enough that most routine civil functions could be carried out.”[ii] While Quinlivan’s study included peaceful post-war occupations, counterinsurgencies, stabilization, humanitarian and peacekeeping operations, he was the first to advocate that force ratios based on a country’s population were the best predictor for a successful stability operation.

For a population the size of the US at (rounding up to) 350M people, that requires 7M troops. Even dropping the 20% of people who don't live in or around big cities, you're still looking at 5.6M troops, and a fuckload of uncontrolled / uncontrollable land perfect for counter insurgency tactics. Given that the us has about 2.12M active and reserve troops, they'd have to more than double their ranks to be able to stand a chance at containing the urban areas of the US, and that's not counting anything else they need to do.

I don't have a contribution because I do not live in the US but I read about martial law on Wikipedia and found this funny:

Probably terrorize everyone with explosive drones.

Like any day when a non-white American is unlucky enough to have to deal with the Police

"As long as it happens to them, I'm okay with it!" - white Americans

"Fuck you, got mine" is one of the true, active and practised American values today

And everyone has guns. The only upside to that..

Short, simple, truth.

Let's not forget as well that not all those active troops are going to likely to be deployed as they're responsible for logistics and tech of the military which would be needed even more in such a situation.

Yep! I may not have been explicit enough, but yeah, the 5.6M troops is solely for peacekeeping, none of the ancillary work that has to be done in order to keep materiel flowing to where it's needed.