Arthashastra
स्व सैन्य व्यायाम योग्यं परस्य अयोग्यं वर्षति यायात् ॥
or he may march during the rains against a country which is suitable for the manœuvre of his own army and which is of the reverse nature for his enemy's army.
english translation
sva sainya vyAyAma yogyaM parasya ayogyaM varSati yAyAt ॥
hk transliteration by Sanscript