[R]ight, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.But this quotation does not express the idea that "might makes right"; when there is an imbalance of power, there is no right; right "is only in question between equals in power."
This is an important passage, because it states a descriptive truth about the world: normative values are applicable only between equals; when there is an imbalance of power, there are no normative values at all, no "right", just the strong exerting their will upon the weak. Might does not make "make" anything; might is might, no more, and, more importantly, no less. Anyone who reads this statement as "might makes right" has failed to comprehend the meaning of ordinary language.
The Athenians are stronger than the Melians, and demand tribute on that basis. The Melians object on various normative principles, refuse to submit, and are conquered. Quelle surprise. Of course, Athens is later conquered by Sparta, but who are they to complain?
That "the strong do what they can and the weak suffer what they must" is a truth about the world, true whether you or I like it or not (I don't like it much myself, but there it is), and anyone who denies this truth has failed to draw basic conclusions from not only the study of history but also the reality of police and prisons. Normative values must take second place to truth: No matter how much you deserve to walk on the Moon, you must first learn rocket science to get there. Likewise, to even talk about notions of "right", you must first achieve parity of power.
Philosophers, especially ethical philosophers, are sometimes said to long for an argument so compelling that one's head would literally explode if it were not accepted. Such a desire is, of course, a longing for power, not truth. Of course, the real truth does — if you do not believe it — make your head explode, or literally gets you eaten by a tiger, smashed at the bottom of a cliff, or conquered by Athens.
When I was a Cub Scout, lo, these many years ago, I was taught a short rhyme: "Here lies the body of Jonathan Day / Died for insisting on his right of way." If you walk in front of a speeding car, it matters not in the least that two thousand years of ethical philosophy and legal reasoning make you absolutely 100% right and the driver 100% wrong: You will be dead and he won't be.
The lesson is simple: The Melians were stupid. They were "right", and they ended up dead. The better strategy would have been to suffer what they must, gain power, and negotiate as equals. Athens too failed to heed the advice Thucydides puts in their mouths. Exactly equal parity is not strictly necessary; it is necessary only to have enough power to make negotiation rationally desirable. Had Athens and Sparta negotiated the "right" as equals or near-equals when their power was near parity, The Athenians might well have avoided their own conquest.
If you want to talk about right, first acquire equality of power. If you have power, be careful: your opponent might not be as weak as he seems.