Kautilya's Arthashastra 7.13.19

7.13.19

vṛddhamitro hy amitroddhārī pārṣṇigrāham ucchindyāt, netaraḥ svapakṣopaghātī //

When there are two kings, one engaged in destroying a friend, and another an enemy, he who attacks the rear of the former gains more advantages; for one who is engaged in destroying an enemy will have the support of his friends and will thereby put down the rear-enemy, but not the former who is engaged in destroying his own side.