Treaty on European union 42.7

If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with Article 51 of the United Nations Charter. This shall not prejudice the specific character of the security and defence policy of certain Member States.

Commitments and cooperation in this area shall be consistent with commitments under the North Atlantic Treaty Organisation, which, for those States which are members of it, remains the foundation of their collective defence and the forum for its implementation.

There's also the point that countries with close economic ties have more to lose by going to war with each other.

It's a necessary component of this sort of idea, but clearly it's not sufficient. I mean, we have had Russia in war with Ukraine for about a decade now, beginning at the annexation of Crimea, even though they had fairly good trade relations, even if they were hurt once Maidan happened.

I think that one needs strong economic ties, but it seems to only truly work with democratic societies.

A point fully disproven by Putin by now.

I don't think that's right.

Economic interdependence is a deterrent against war, not an absolute barrier.

If you can easily swap trade partners for example, it's less of a deterrent. Which is one of the reasons why EU has gone beyond simple import/export agreements.

Kinda, but NATO is (imho) more about external threats. I think part of the EU's founding vision was to prevent further intra-European wars, in which it has been largely successful.