Kautilya's Arthashastra 7.7.9

7.7.9

evaṃbhūto vā hīnaśaktipratāpapūraṇārthaṃ sambhāvyārthābhisārī mūlapārṣṇitrāṇārthaṃ vā jyāyāṃsaṃ hīno balasamād viśiṣṭena lābhena paṇeta //

In view of marching for the purpose of exacting some expected revenue to be utilised in recouping his own strength and resources, an inferior king may request of a superior the help of the latter's army for the purpose of guarding the base and the rear of his territory in return for the payment of a greater share in the profit than the strength of the arm supplied deserves. The king to whom such a proposal is made may accept the proposal, if the proposer is of good intentions; but otherwise he may declare war.