Well, the plan hinges on taking the enemy capital by surprise and forcing their surrender. If (big if) the plan goes as expected and they capture or force the king of Kannauj to submit, they won't need to worry about the enemy on their rear.
Access to pre-Collapse building techniques I assume. Wouldn't be surprised with people with the expertise to build those managed to survive and pass on those skills even if the ancestors of the Western and Middle Villages didn't.
I mean, it's what literally worked for Qin and the Roman Republic. The reason why they usually won wars despite horrible losses was that their conscription system was robust enough to replace those armies and grind the enemies down by throwing more bodies.