Luckily there are other defensive alliances in place.

The broad advantage of having a "felt of society" —mutually overlapping circles— as opposed to a "fabric of society", is that the latter is much easier to tear.

European Union article 42(7) for example. But it has not been very well developed since there was little need with Nato around.

Not well developed would be to our advantage if the casus foederis were in a grey area.

(It's somewhat :lolsob: to note that Thucydides already distinguished between "stated reasons" and "real reasons" behind a conflict; remember "Remember the Maine!"

https://en.wikipedia.org/wiki/Casus_belli#:~:text=Proschema,... )

EDIT: looks like there may not even be a need to be part of an existing mutual defence alliance; sending troops to take part in Operation Arctic Endurance might suffice for an Art 51 response to invasion?

https://opil.ouplaw.com/display/10.1093/law:epil/97801992316...

3(a) and 3(d) refer to: https://iilj.org/wp-content/uploads/2016/08/General-Assembly...

Exactly what Putin wants.