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.

Well, I'm not a committed anarchist but I'm sympathetic and within range of being convinced, as a caveat. But I'll chime in for the sake of conversation.

I think it has mostly to do with what I think would happen if the invader won. Invasions are not usually contemplated by well intentioned states, so on the whole I'd recommend joining the defense. If you're a pacifist you can still help a lot with mutual aid, or by doing war journalism to bring awareness to potential allies.

If I think the current state and the invader are both equally bad, perhaps best to seek a way to exit the situation and prepare for a planned return in greater numbers if and when the smoke clears.

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.

Like others have said in this thread, it depends on the circumstances.

In addition to that, I would say that the way I think of my own anarchism means that I don't hold myself to strict standards because I'm not trying to build an anarchy, but more anarchists. By that, I mean that in practice, the world is quite far off from being an anarchist world and that if I were to adhere to dogmatic principles, I wouldn't make much progress.

It's a lot less drastic than fighting in the military, but an example of one of the compromises I make is that I have done some activism at the local level regarding access to health and social care services (including accessible housing). I don't necessarily think that these functions are best fulfilled by the state, but also, I can't envision a world without the state (in this domain at least). But also, I don't feel bad about my failure of imagination, because anarchism, for me, is about letting go of grand narratives about myself and the world, recognising that I am such a tiny part of the bigger picture and that I can't do this alone. Along those lines, it's a pragmatic choice to push for better socialised services, even if that means enlarging the state, because it'll help give voices to people who I want to have a say in the world.

It doesn't feel like a compromising of my principles, but a more genuine way of honouring them. Something I like about anarchism is that it's messy, and it's a process. As a framework, it's helped me to grow a lot, and I feel like I need to be open to situations that challenge my principles because I know I'm a better anarchist now than I was a year ago, and dogmatically sticking to certain rules or principles would feel like I have decided I am currently the best I'll ever be. So even though I don't like the thought of it, I need to be open to the principle of having to fight, if it were necessary, as well as possible needing to resist fighting (like Israelis who would rather be imprisoned than be complicit).