The idea is that these socioeconomic orders are global. Capitalism today is global. Even if a country today tries to do not-capitalism, it still must engage in the capitalist sphere, doing trade with them, using money system, debt, and producing purely for the purpose of selling. These are aspects of capitalism we stuck with until the global order isn’t capitalism.

So communism would not come about unless it is global. In which case the question of “other countries” would not apply. You can assume that for whatever reason, a breakaway bunch decide to revert back to capitalism, but that would not go well. Why? Why would anyone whose needs are fully met and their entire time is only spent doing things for their own interests and community decide “I actually wish I had to give most my time to a capitalist in exchange for money that allows me to buy my needs”? For one, money wouldn’t exist in communism, so that part would not even appeal you. Capitalism only has the upper hand because it is already the global system. Once it is overthrown, it is the reverse.

Obviously a society will put guards to deal with lunatics wanting to destroy society for ideological reasons (trying to restore capitalism). It would be in their interest to do so.

I hope I answered your question. Unless your question was “how do we prevent resistance during the revolution / transition”?

What if i told you that marxist theory is not some isolated idea from a random guy but the conclusion of a scientific analysis of economic history through the lens of dialectical materialism, and built on top of the works of many other people?

An easy way too look at it is that marxism is for economics what darwinism is for biology.

The best read on this is “Dialectical and Historical materialism” by Stalin.

As marx put it, the only way capitalism would survive is by keeping an infinite growth. Tech is a prime example of that phenomena, where new needs are being created out of thin air: subscriptions, software, etc… Cars, phones have begun to be necessary. That’s how capitalism survives still today: growing more and more by creating new needs for the individual. Except this growth is at the expense of finite ressources, and this is where we’re gonna hit a wall.
Maybe this explains we haven’t seen a capitalist collapse yet. But with today’s ecological concerns, it seems closer than ever