Arthashastra
मार्ग आदेशिकम्, पर भूमि योग्यम्, अरि युद्ध प्रतिलोमम्, अटवी बल प्रायः शत्रुर् वा, "बिल्वं बिल्वेन हन्यताम्ण्" अल्पः प्रसारो हन्तव्यः इत्य् अटवी बल कालः ॥
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.
english translation
mArga Adezikam, para bhUmi yogyam, ari yuddha pratilomam, aTavI bala prAyaH zatrur vA, "bilvaM bilvena hanyatAmN" alpaH prasAro hantavyaH ity aTavI bala kAlaH ॥
hk transliteration by Sanscript