Solarpunk

Solarpunk and its Anarchist ideals are a pretty direct threat to virtually every other societal system. Fascists, Capitalists, and Authoritarians all either see it as an enemy to starve out, or even directly attack in an attempt to prevent its spread, as was seen during the Russian Revolution with Makhno's Black Army, and the Spanish Civil War.

As long as there are still state governments with military might at their disposal, a Solarpunk society is likely going to need to be able to defend itself against them. The Anarchists of Spain organized their voluntary militias from the bottom up, where the soldiers themselves would elect someone to lead the group (but with the ability to immediately recall them outside of battle if they didn't perform their duties to the satisfaction of the soldiers who elected them), and those elected leaders would then elect a commander to help organize them more effectively, with the same recall rules applied. It seemed to work out quite well for them.

However, there may be a point in time where the advantages of the alternative solarpunk system are too apparent to ignore, which could spark mass-civil disobediance or even revolution in those other countries causing their governments and capitalist economies to collapse, hopefully then leading to them joining in with their own solarpunk movements. Were that to eventually happen the world over, the need for militias to exists would likely dwindle.