Proposed Canon
Warlords may claim a city that is not currently controlled by any Side.Erf-b1-p078 This act places the city under the command of the Warlord's Ruler.
Barbarian Warlords can create a new side by claimingErf-b1-p078 or rebuildingErf-b1-p099 a city on a Capital Site.
It is also implied that any warlord could, theoretically, claim a city. Due to having Loyalty to the side that Popped them, this could be a way for a side to create a number of puppet sides
It is possible that all Commanders, including Casters, may claim a city, or that this can only be performed by Chief Warlords.