Kautilya's Arthashastra 7.4.15

7.4.15

yadā vā paśyet "vyasanī paraḥ, prakṛtivyasanaṃ vāsya śeeṣaprakṛtibhir apratikāryam, svacakrapīḍitā viraktā vāsya prakṛtayaḥ karśitā nirutsāhāḥ parasparād vā bhinnāḥ śakyā lobhayitum, agnyudakavyādhimarakadurbhikṣanimittaṃ kṣīṇayugyapuruṣanicayarakṣāvidhānaḥ paraḥ" iti tadā vigṛhya yāyāt //

When a king finds that his enemy has fallen into troubles; that the troubles of his enemy’s subjects can by no means be remedied; that as his enemy’s subjects are oppressed, ill-treated, disaffected, impoverished, become effiminate and disunited among themselves, they can be prevailed upon to desert their master; that his enemy’s country has fallen a victim to the inroads of such calamities, as fire, floods, pestilence epidemics (maraka), and famine and is therefore losing the flower of its youth and its defensive power,--then he should march after proclaiming war.