Kautilya's Arthashastra 7.13.16

7.13.16

mitrābhiyogī hi sukhenāvāptasiddhiḥ pārṣṇigrāham ucchindyāt //

When there are two enemies, one engaged in attacking a friend and another an enemy, he who attacks the rear of the latter gains more advantages: for one who has attacked a friend will, after easily making peace with the friend, proceed against the rear-enemy; for it is easier to make peace with a friend than with an enemy.