NATO exists solely to address the problems created by the existence of NATO.

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.

https://en.prolewiki.org/wiki/North_Atlantic_Treaty_Organization

The simplest and most positive approach I've found that works with people who genuinely want revolution is to ask them who the allies and enemies will be if a new socialist state pops up anywhere in the world.

Getting them to walk through it in their own head, to have the lightbulb moment of "Oh it would nato" and "Oh their best chance for weapons and ammunition would be Russia, Iran, DPRK, China, etc".

When they realise who the biggest enemy of the newly socialist country would be they start to get it.

Further to this - you can make the example a nato country itself. Ask them what happens to Germany if it flips socialist tomorrow. Think that nato is going to be supplying them weapons and ammunition? Or sanctioning them? Or worse still invading them?

The answer is the same over and over again and it doesn't matter if they're currently a nato member or not. It happens in all scenarios because they're the enemy of socialism.

If they don't genuinely want socialism then the best I can do is suggest pointing out almost everything nato has ever done has been offensive. Where's the defensive?