Kautilya's Arthashastra 9.2.8

9.2.8

mārgādeśikam, parabhūmiyogyam, ariyuddhapratilomam, aṭavībalaprāyaḥ śatrur vā, "bilvaṃ bilvena hanyatāmṇ" alpaḥ prasāro hantavyaḥ - ity aṭavībalakālaḥ //

When he thinks that the army of wild tribes is living by the same road (that his enemy has to traverse); that the road is unfavourable for the march of his enemy's army; that his enemy's army consists mostly of wild tribes; that just as a wood-apple (bilva) is broken by means of another wood-apple, the small army of his enemy is to be destroyed, then is the time for engaging the army of wild tribes.