There's a growing concern that the US would not defend its allies, or worse, join Russia in a European war. So, right or wrong, Europe is thinking of building

There's a growing concern that the US would not defend its allies, or worse, join Russia in a European war. So, right or wrong, Europe is thinking of building an alternative. It would require not only military, but also political coordination. The options are to do it within, or outside the EU. Within the EU, it is somewhat straightforward. As a sibling comment says, there is already a vague commitment to mutual defence. There is a forum for political discussion, common diplomacy, mechanisms for coordinating industrial policy, the works. And any changes could be seen as incremental strengthening of an existing thing. To do it outside of the EU, most of all, makes the original concern explicit - it is saying US has left the NATO. This is in no-one's benefit. Even ambiguous US participation is valuable to Europe. The other downside is that, as is, a EU "NATO" can't easily include UK and Norway, both valuable allies. Which is maybe what Norway wants to address here.