Kautilya's Arthashastra 7.13.43

7.13.43

kṛtsne ca maṇḍale nityaṃ dūtān gūḍhāṃś ca vāsayet /
mitrabhūtaḥ sapatnānāṃ hatvā hatvā ca saṃvṛtaḥ //

he should send messengers and spies to reside in each of the states composing the Circle and having again and again destroyed the strength of his enemies he should keep his counsels concealed, being friendly with his friends;