For me anarchism not about opposing a particular state (eg. your own state, some enemy state, etc) but about opposing every state and "the state" in general.

From this perspective there's always nuance when one "state attacks another". What are the states? What is their relationship? Why is there conflict? How does this affect me and the planet more generally?

And so someone's behavior could vary greatly depending on the situation. If you're living in nazi germany and your "country" is attacked, don't collaborate with the nazis. On the other hand if you're invaded by nazis, it's probably worthwhile to fight back - perhaps in the military or perhaps not. In many cases both states are terrible and it's a struggle to just survive their violence. I'm using nazis at the typical extreme example but most states are built on similar principles: fascism, xenophobia, patriarchy, colonialism/imperialism, etc. So many conflicts fit into this general framework.

TL;DR: It depends on the situation.

You probably should be reading about the Spanish civil war instead...

I guess that would highly depend on how the specific national army is managed. In some cases they allow largely independent brigades to be formed. But in most cases, anarchists would probably opt for a more partisan like defense of the places where they live instead of being send to some far away front as cannon fodder.