Kautilya's Arthashastra 7.7.16

7.7.16

parānīkasya pratyanīkaṃ mitrāṭavīnāṃ vā, śatror vibhūmīnāṃ deśikaṃ mūlapārṣṇitrāṇārthaṃ vā samo balasamena lābhena paṇeta //

When a king proposes peace to another king of equal power on the condition of receiving the help of the latter army strong enough to oppose an enemy's army, or to guard the front, centre, and rear of his territory, or to help his friend, or to protect any other wild tracts of his territory in return for the payment of a share in the profit proportionally equal to the strength of the army supplied, the latter may accept the terms if the proposer is of good intentions; but otherwise he may declare war.