Why would Finland and Sweden not establish a joint defense alliance instead of joining NATO?

Why would Finland and Sweden not establish a joint defense alliance instead of joining NATO?

We already have one of those.

But that was based on the assumption that Russia wanted good relations with theie neighbours. The USSR was like that; sometimes rowdy but we basically had each other figured out. The key was simply not to be a threat to them.

Now, it's obvious that Russia wants outright hostile relations, so we're adapting to that. We're about to join a bigger club, since Russia will attack other countries. Not being a threat is useless today; Russia sees that as an invitation to invade.

Remember, we've had them next door for 1,000 years. We know when it's time to be prepared.