Kautilya's Arthashastra 7.3.13

7.3.13

saṃhitaś cet "paraprakṛtayo lubdhakṣīṇāpacaritāḥ pratyādānabhayād vā nopagacchanti" iti paśyedd hīno 'pi vigṛhṇīyāt //

When a king in peace with another finds that greedy,.impoverished, and oppressed as are the subjects of his ally, they do not yet immigrate into his own territory lest they might be called back by their master, then he should, though of inferior power, proclaim war against his ally.