I don't really understand how an economic system has anything to do with a military body? The NATO purpose isn't to spread capitalism, if it was then why is a country like Portugal a NATO member given it values socialist ideals? Would you really consider Portugal as a US colony?
Now vs NATO, a stronger argument could be made around Chinese imperialism and colonialism because Chinese influence is dominated by economic strength as opposed to military strength. Compared to us the Chinese are literally defined by their economic system and China isn't even a democracy to enable its own people to choose let alone others.
I guess as a socialist you're not bothered by that though? Would you be bothered if China sliced apart the UK and made it into a communist colony? I doubt it because bring on the hammer and the fist as long as he's a socialist....
The difference with NATO is countries have a choice and even then that choice is limited purely to what NATO military capability can do for the goals of that country. NATO enables the people of a country like Romania to make a choice to join the EU for example without fear of consequences due to the threat of Russian aggression.
In this example the fact I know you disagree with the EU's own goals is the problem in your argument because that still has nothing to do with NATO.