No economic or political system can be shown to be practically achievable before it's been achieved. If you don't think the following examples are examples of genuine socialism/communism, then that's not an argument against the ideology.
We've had communists fight alongside other leftists. So revolutionary Catalonia was a functioning leftist space, meeting all the criteria to be called communist (classless/moneyless/stateless). It functioned incredibly well for a year before it was invaded.
If you want a longer, but smaller example, Red Vienna existed for about 2 decades and was a fully functional socialist space that improved worker's lives before being outlawed by a regime change.
If your position is that imperialist capitalist nations will always invade/outlaw well-functioning socialist/communist systems, you can't know that for sure, but it's definitely a possibility. That doesn't mean the entire idea is worth throwing away.
It won't always be the same ones so I'm unsure of what you're asking. Which ones invaded the spaces I listed before?
For Revolutionary Catalonia it was the Nationalist Faction who overthrew them. They advocated for, and implemented, a form of national syndicalism that was "fully compatible with capitalism".
For Red Vienna is was the fascists who overturned the socialist policies and returned the city to a state of capitalism, allowing land-leeches and other bourgeoise to return to continue exploiting the working class.