Kautilya's Arthashastra 9.2.5

9.2.5

"prabhūtaṃ me mitrabalaṃ śakyaṃ mūle yātrāyāṃ cādhātumṇ" "alpaḥ pravāso mantrayuddhāc ca bhūyo vyāyāmayuddhamṇ" "mitrabalena vā pūrvam aṭavīṃ nagarasthānam āsāraṃ vā yodhayitvā paścāt svabalena yoddhayiṣyāmiṇ" "mitrasādhāraṇaṃ vā me kāryamṇ" "mitrāyattā vā me kāryasiddhihṇ" "āsannam anugrāhyaṃ vā me mitramṇ" "atyāvāpaṃ vāsya sādayiṣyāmi"
- iti mitrabalakālaḥ //

When he thinks that the strong help he has in his friend can be made use of both in his own country and in his marches; that he has to be absent only for a short time, and actual fight is more than treacherous fight; that having made his friend's army to occupy wild tracts, cities, or plains and to fight with the enemy's ally, he, himself, would lead his own army to fight with the enemy's army; that his work can be accomplished by his friend as well; that his success depends on his friend; that he has a friend near and deserving of obligation; or that he has to utilize the excessive force of his friend, then is the time for the enlistment of a friend's army.