Kautilya's Arthashastra 7.5.1

7.5.1

tulyasāmantavyasane yātavyam amitraṃ vety amitram abhiyāyāt, tatsiddhau yātavyam //

WHEN two enemies, one an assailable enemy and another a strong enemy, are equally involved in troubles, which of them is to be marched against first?

The strong enemy is to be marched against first; after vanquishing him, the assailable enemy is to be attacked, for, when a strong enemy has been vanquished, an assailable enemy will volunteer of his own accord to help the conqueror; but not so, a strong enemy.