Kautilya's Arthashastra 7.7.28

7.7.28

paṇitas tasyāpakārasamartho vikrameta, anyathā saṃdadhyāt //

When a king of superior power falls into troubles owing to the weakness of the elements of his sovereignty, and requests of an inferior king the help of the latter's army in return for the payment of a share in the profit proportionally equal to the strength of the army supplied, the latter, if powerful enough to retaliate, may declare war and if otherwise, accept the terms.