Kautilya's Arthashastra 7.16.2

7.16.2

viparyaye kṛtapratīkāro yāyāt //

IN view of causing financial trouble to his protector, a powerful vassal king, desirous of making conquests, may, under the permission of his protector, march on countries where the formation of the ground and the climate are favourable for the manœuvre of his army, his enemy having neither forts, nor any other defensive works, and the conqueror himself having no enemies in the rear. Otherwise (in case of enemies in the rear), he should march after making provisions for the defence of his rear.