In Endless Legend, the militia units can provide some additional military power when countering a monster attack or invasion from another faction.
However it's difficult to determine how far and when they will help in a battle. I've seen militias from 2 cities participate in an attack, while in another game I've been unable to get the militias to provide assistance in a relatively close battle. It seems like tech, buildings, population, or movement may have an impact on the militias assistance range, but its not precisely clear what or how militia assistance is determined.
I found this quote on an wiki article about battles:
The Militia is a group of units generated by a City. They can only be used when a battle takes place and overlaps the city they are in. They can be improved by various research technologies.
But I have seen multiple battles with the city in the center of the battle where the militia does not help.(is this a bug?)
How far and in what situations will militia participate in a battle?