Most of the time people are pretty cool, but when people aren't cool all the cool people need to be like "hey, that's not cool, so be cool or you need to leave" and thus the coolness is enforced.
Additionally because of game theory, a "rat" will always exploit everybody's chillness. It'd be great to not spend a cent on the military but Russia is a rat that uses this opportunity to invade its neighbors. Inevitably you will have to invest in some kind of law enforcement.
Anarchist movements have military as well, in need they can pick a commander to lead them whose power can be dissolved at any time even without finding an alternative commander. Commanders power is not enforced and anyone can not listen to them. These armies where most efficient in the past, but they were most of the time defeated because both right-wing and so call "left-wing" communist fight them. This is what happened during anarchist movement in Spain, which no one seems to talk about.
I'd be more concerned how to disarm the military. Giving someone power, and them willingly giving it back never works. There have been times when a country had no effective military and mere civilians banded together but these were very rarely effective combat forces.
Exactly, that is why no anarchist believes in giving someone power that they can't take back at any moment. That is exactly the anarchist point of view, you don't give other people power, no matter what they say they will use it for.
Power of a commander can be purely on a voluntary basis, many native american tribes lived exactly this way. Most of the prehistoric humans lived this way. You can choose to follow someone more experienced, but that is purely your choice and at any moment you can just simply step-away if you disagree. No one is saying you can't look for inspiration for making decisions in others, but you should never agree to follow someone whatever choice they make.
All armies are made of mere civilians, we are all just people. It is just that some are more experienced then others, for example, by having formal military training or being a veteran. In those situations, people naturally listen to those with more experience, as long as they seem reasonable. Furthermore, you can have formal combat training in societies without rulers as well. So having mere civilians banded together is not a trait of anarchist society.
In the modern era weapons and technology require the coordination and organization of millions of men for a single weapon or consumer product. From the armor, steel, engine, fuel, cannon and shells of a tank it seems too complicated to organize without a rigid government (in fact multiple governments as many parts are imported).
I think anarchism could have only existed prior to the industrial era, where much of your technology, development and manufacture was local to you. From your local baker, blacksmith and watchmaker expecting good faith is a reasonable assumption, however today everything involves such great planning and organization, a powerful centralized entity inevitably becomes necessary. An entity to also protect every step of an increasingly complex and fragile supply chain.
A "primitive" vs "modern" economy is often metaphorically represented by a donkey and jet engine by economists. A jet engine is much faster and more powerful than a donkey, but a single malfunction in its myriad of components leads to explosive decomposition. I'm also concerned that the public would renain unconvinced to regress technologically, most people are willing to sacrifice personal freedom for greater quality of life. I suppose you could join the Amish as an opportunity for a self-sustainable anarchic government.
Spain was run without government during anarchist revolution in 20th century and their production and technological advancement increased, since there was no one on the top taking all the profits that otherwise would be used for investing in the community and industry. Quality of life is always better in free societies, then when you have to listen to your boss, work long hours and struggle to pay the bills. As far as I know, once Europen children were raised in native amercian egalitarian societies, they would never want to go back to cities and work long hours under someone else's rule.
I don't know much about Amish, but I got the feeling that they are patriarchal society, which is exactly the opposite of a free anarchist society where no one rules over anyone else.