General strikes are illegal in the US.

It’s not illegal to strike on a date with other people. It’s illegal for unions to call for a “general strike” because it’s considered them calling a strike on behalf of other non-union employees for other businesses.

Also, jobs can fire workers on the spot for participating in them

Not always, (though yes, it would probably be likely for many people) since they can use things like sick/vacation days conveniently timed right, or if they’re backed up by a union, they might have a contract that helps to prevent at-will firing without certain specific causes, excluding striking.

However, if enough people strike, it’s kind of hard to enforce coming into work via firings, as it’s similar to if an entire unionized company goes on strike. What are you gonna do? Fire every single worker and shut down for good the next day because the only person running every single operation is the remaining CEO?

even if the workers are part of a union and the union want to participate.

As long as the union doesn’t say “this is a general strike” and just says “we are striking on this date for better working conditions”, and that date happens to be the same day other unions are striking, it’s legal. There is no law preventing different unions from striking on the same dates, and it would take very long for any legal process to try and make that claim before the strike has already occurred.

national guards have been sent in to shut down general strikes in the past.

This is the most likely outcome in my opinion. However, it’s still kind of hard to actually enforce the end of a general strike. It’s one thing to arrest someone, or to stop them from doing a given thing, but it’s another to forcibly remove people from their homes and make them work no matter their condition or reason.

Essentially, I’m saying it’d be messy.

Doing it multiple days? You realize most people live paycheck to paycheck? Nobody wants to tell their kids they’re going to be homeless.

This is the biggest hurdle, though there is a degree to which it can be mitigated, at least for a little while. For example, there are a lot of people with backyard and community gardens, small businesses with stockpiles that are willing to support their community as we’ve seen with the current situation in Minnesota, not to mention that if the situation got bad enough you’d probably just see people stealing from their nearest billionaire-owned store because fuck it, why not screw them over more?

To clarify, I’m not like, disputing your actual overarching thesis here, or saying a general strike is easy or likely to succeed, I’m just saying it’s not entirely impossible :)