Kautilya's Arthashastra 7.18.30

7.18.30

tair bhūmyekāntarā vyākhyātāḥ //

Those neighbouring kings who can be servants to the conqueror are a neighbouring king under the apprehension of an attack from a powerful king, one who is situated between the conqueror and his enemy, the rear-enemy of a powerful king, one who has voluntarily surrendered one-self to the conqueror, one who has surrendered oneself under fear, and one who has been subdued. The same is the case with those kings who are next to the territory of the immediate enemies of the conqueror.