Discuss whether kings in early states were invariably Kshatriyas.
And in this text Ksatriyas or the warrior caste only can be the kings.
However we find that it was not so, it was anyone who could muster support and resources, and rarely depended on birth as a Kshatriya. For example Mauryas, there ancestry are hotly debated. Buddhist text suggests that they were Kshatriyas whereas Brahmanical text desceibes them as of Low Origin. The Shungas and Kanvas, the immediate successors of the Mauryas, were Brahmanas. Other rulers, such as the Shakas who came from Central Asia, were regarded as mlechchhas, barbarians or outsiders by the Brahmanas. And the Satavahanas claimed themselves to be Brahmanas.
Thus we can clearly see that it was not necessary for a king to be Kshatriyas, it was fairly opened to anyone who could muster support and resources.