Assyria a Superpower?

Between the years 934 and 827, Assyria underwent a number of military campaigns, expanding its territory and redefining its frontiers. What stood out to me, was that Assyria carried out a "policy of terror" towards it neighbors, and those who refused to submit were forced to pay an annual tribute (maddattu). Ashurnasirpal II was the main user of this type of policy, allowing his neighboring cities to maintain internal autonomy as long as they paid their annual contributions. This set up was not completely a loss for the neighboring cities, however, as they were guaranteed protection and sometimes even a share in booty. What surprises me, however, is how powerful Assyria must have been, if theses facts are true. It is rare for a superpower to be so terrorizing and powerful that a kingdom would agree to pay an annual contribution as an autonomous state. Of course this sort of ruling was bound to be received with disdain, and soon enough, allies formed against Assyria. However, it truly is remarkable how big of an army Assyria must have had, and how much power they resided over these kingdoms, that independent kingdoms would agree to such a "diplomatic treaty."