“Right, 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.”

—Thucydides

You can't honestly attribute that quotation to Thucydides. The idea appears in his work, but he specifically attributes it to other unnamed parties. It receives this immediate response:

As we think, at any rate, it is expedient — we speak as we are obliged, since you enjoin us to let right alone and talk only of interest — that you should not destroy what is our common protection, the privilege of being allowed in danger to invoke what is fair and right, and even to profit by arguments not strictly valid if they can be got to pass current. And you are as much interested in this as any, as your fall would be a signal for the heaviest vengeance and an example for the world to meditate upon.