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?
Best Answer
When you right-click an enemy army to attack them, before you release the right mouse button, you get an overlay of where the battlefield will be.
This could be tactically important if you e.g. need to keep the high ground or wish to engage them from a direction that suits you best.
If a city falls in the designated battlefield tiles, its militia will join the fight.
As was mentioned in the comments, you need to have militia ready, that have not already fought this turn.