The Anarchist rebellion in Spain for instance was not only instantly crushed by all sides (Fascist, communists, liberal democracies) but it has been written out of history too. You hardly ever learn about it, except if you read Anarchist books.
The anarchist rebellion was the only anti-Franco side that managed to win some battles, it's demise happened because liberals and Stalinists preferred to betray anarchists and then get steamrolled by Franco.
One thing anarchist societies do not seem to be particularly effective at is war. Spain proved that they can do the rest though.
If the militia had been armed properly, maybe it could have done better. But we probably need a global revolution or solidarity to take place since smaller local revolutions can be crushed by larger powers.
They seem to actually be very efficient per capita, it's mostly that anarchism never really was more popular than other "radical" ideologies like Marxism-Leninism or fascism. It was almost always less popular than those even where it was popular.