There’s no utopian vision advocated for by Communist philosophers. They talk exactly about how this would come through. So yes, they speak about it as an achievable and feasible thing.
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”?
Bob: “Guys… if we could get everyone in the whole world working together to efficiently organize labor and the allocation of resources, there would be no more poverty”
Alice: “Wow Bob, that sounds amazing! How do we make that happen?”
Bob: “Uhh… how many bullets do we have?”
