I agree with what Darvin said about adding supplies. But would they need to consume resources such as gold and food? or just be there? what consequences for the troops without supplies? do they lose health, maybe stamina? Something else is maybe allow forward bases to make units normally but you have to send the supplies to make the troops via convoy?
An incentive for this could be that supply wagons are faster or use less supplies then simply marching an endless line of troops to the defenders stronghold from yours. This would allow the defender or allies to cut off the attackers supply route ending immediate reinforcements and starving the army.