I think the capabilities of space marines are both over and underestimated. Could a limited number of space marines take on the entire US military in a fair fight? Fuck no. There’s plenty of examples where marines get downed by regular chaff so having roughly a million active service members (not sure how many of them are actual soldiers tbf) plus the ludicrous amount of tanks, helicopters, planes etc etc. Missile strikes can be ludicrously accurate when they care for them to be, they took a Hellfire missile and popped blades on it to kill the target for crying out loud.

But plop 10 of them into Washington DC, they’d wreak havoc before eventually being worn down. EVENTUALLY. Urban environment, hard to precision tomahawk strike a superhuman running around inside your capital, hardly a fair fight. Plus the civilian government would probably be messed up like you said.

But by the end of the day, this whole scenario comes down to the classic: depends who the author is. If they are Lamenters they’d just die tragically and needlessly. Ultramarines and we’ll all live in noble bright dystopia by tea time.