If you allow anyone to declare independence then Europe might split back apart into hundreds of small provinces. For example, there were times when major Greek and Italian cities were independent states, what if they will want to return back to that time? In this case an external enemy will conquer those small states one by one like Pacman eats dots.

Why do you think a military alliance requires a political one? They can join e.g. NATO if they are afraid of Russia.

They will not be able to meaningfully contribute to such military alliance - every state having single fighter jet? Or a few tanks?

And if they pool money to form an army - how will be the funding determined? Some states will want more defence, some less, some will prefer naval power, some airpower . . .

if those people can't agree on anything to such extent that they want to be entirely separate, how would they agree on anything when they would have even less incentive to cooperate? See how dysfunctional EU seems at times today, and this would be way worse . . .

Especially when you start considering other forms of warfare, like trade. Solving all problems leads right back to something resembling either superstate or close coalition of large states.

They will cooperate if they have a threat of invasion, they won't cooperate to have a large standing army - and that is a good outcome. For example see mutual defence of Switzerland cantons before federal state.

How will they contribute to NATO then? Your point was that they don't need political alliance to be in a military one, my point is that this is practically impossible.

Also good luck with making an army from nothing just in time before invasion.

Edit: and you are still ignoring weakness of such states on economic level.

History shows it was possible in cantons of Switzerland, Habsburgs couldn't conquer them, despite no federal authority. There are still examples of small countries in military alliances today - I don't see what is the problem with contributing.

Economics is not a problem in a modern world with a free trade. Small city-states, that embrace free trade, fare well. There is even an possibility, that trade liberalization leads to political disintegration: https://www.nber.org/papers/w6163

Imo Switzerland is poor example and free trade cannot be guaranteed for small states in today's world but I guess we will not convince each other in this limited debate . .. Let's agree to disagree.