The main issue I see is how an anarchist society could defend itself against a more united outside threat. Wouldn’t an invading force with an established military hierarchy have a significant advantage over a group of individuals who’ve banded together?

Why are these people invading?

What do they plan to take?

Do they plan to pillage like vikings?

Is it a one time theft of our Priuses and iPhones?

Do they plan to establish a government from scratch?

There are no existing government offices to overtake.

There is no central authority to surrender on behalf of the society as a whole. Will they occupy every town?

Do the people in this invading country support this war?

Why do they support it? 

Do the industries of the invading country support this war, given the loss of laborers and disposable incomes that it will entail?

What about local producers of the object of desire of the invaded country? Do they support the sudden influx of stolen goods that will undercut their prices?

Will the people of the invading country continue to support it as more and more flag draped coffins go back?

If these people are irrational warmongers, why does it matter if we have a government or not?

The anarchist society, do they not have their own defense contractors/mercenaries? If there’s an irrational warmongering state across the not-border, why was nobody prepared?

If the warmongering state will invade regardless of all of these concerns, how would us having a government deter them?