Thucydides' conception of the function of power in the relations of states was not a theory, and it was not inaccurate. It was a clear and unprejudiced understanding of the relations of states at the time.
It was not "amoral"; rather, "moral" in the sphere of state relations was the pursuit of interest (not the "pursuit of power"). These ideas would later echo, most famously, in Machiavelli.
This conception of state relations is no more obsolete today than Adam Smith's invisible hand. Only the pursuit of interest can sustain a state. In The Melian Debate, the Athenians take Melos not because they seek to become more powerful so much as they seek to not become less powerful. This is an extreme example. But Thucydides' observations still hold water today; the Gulf War is perhaps the most perfect recent example of a nation going to war for its interest. Because America would become a weaker country if Kuwait were to fall, because America's gas prices are of central importance to its economy, intervention became the defense of interest.
That Americans were shipped thousands of miles to kill and to die in the desert for gas prices is immoral in the sphere of the individual; but in the sphere of international relations, it's perfectly moral, and even necessary--this is what Thucydides voiced.
Dash2 scoffs at Thucydides' consideration of "morality (as) nothing but a pretence"; but the rhetoric of the Gulf War centered on "protecting" Kuwait, "saving" Kuwaitis, thwarting "evil", and preserving the stability of the region, rather than the importance of oil.