For anarchism to happen ideally, I believe everyone in the world has to share the same values and mindset for it to work. That foreign army invades, because they don't share the same peaceful and horizontal societial values as we do. In other words, anarchist values has to be planetary wide.

The best example of planetary wide anarchist community is the Fremen in Dune. They still have local cultural differences between the northerners and southerners but they all get along. But in spite of that condition, there is the problem yet again of foreign armies coming to invade, but from a different planet. This yet again requires the need for scaling anarchism to a universal level to prevent the threat of "outsider" invasion.

I would be an anarchist, but because of said reasons, we still don't have the culture yet of wide scale egalitarianism. Having grown up multicultural, I know many cultures are hierarchical and competitive. Unfortunately, with the current paradigm, the threat of foreign armies invading anarchist societies is real.