Ask them what they thought of Vietnam.

How did that defensive pact do any good?

NATO should not be supported because it is an imperial project, quite possibly the largest of all time. If you believe that the vast majority of Western nations are sitting on ill gotten gains, or even the simple tenet that companies/the rich exploit people , I don't know why you wouldn't be amenable to the idea that the countries of NATO have drained the global south of their own wealth via overwhelming collective violence.