Kautilya's Arthashastra 7.13.23

7.13.23

pārśvasthāyī hi yātavyābhisāro mūlābādhakaś ca bhavati, mūlābādhaka eva paścātsthāyī //

When the assailable enemy is capable of retaliation and when the assailant's rear-enemy, capable of augmenting his army and other resources, has entrenched himself on one of the assailant's flanks, then the rear-enemy gains more advantages; for a rear enemy on one of the assailant's flanks will not only become a friend of the assailable enemy, but also attack the base of the assailant, whereas a rear-enemy behind the assailant can only harass the rear.