There’s a lot of questions in there, and I’m genuinely really sorry to say, there’s way more than I can hope to address with the limited amount of time and energy I have, but I think you’re imagining an “anarchist state” or something like that - that’s still thinking with a non-anarchist mindset. There is no country to invade, there’s an amorphous blob of land, which I suppose another nation could attempt to impose itself upon, but in that case, all the working class needs to do is overthrow the new would-be autocracy. Why would a standing military force be more effective than an informal, organized resistance, fighting for their own land? You’re imagining pitched battles and the like, instead imagine trying to occupy land where there’s not really any clear military targets, but everywhere you attempt to impose control, your soldiers end up getting shot, stabbed, or having molatov cocktails thrown on them/their vehicles. Militarism does not protecting the people who live in a country, they’re a tool of the ruling class to fight other nations. This is just my opinion, though - ask ten anarchists, you’ll probably get twenty answers. We believe in creating a better society through consensus, which makes it a little tricky for anarchists to talk about solutions to specific problems on an individual basis.

I’d recommend you check out the anarchist FAQ if you have more questions - https://anarchistfaq.org/afaq/index.html