Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

86. He who thinks otherwise is convicted out of his own mouth. For when you originally invited us, the danger which we should incur if we allowed you to fall into the hands of the Syracusans was precisely what you held before our eyes, and now you ought not to distrust the argument which you thought good enough for us. [2] Nor should you suspect us because we bring hither a force larger than before; for we have to contend against the power of Syracuse. Much more to be mistrusted are they. [3] Without your aid we cannot even remain where we are, and if we were so dishonourable as to make conquests we should be unable to retain them¹, for the voyage is long, and it would be a hopeless task to garrison great cities which, though situated on an island, have the resources of a continent. Whereas these men are your nearest neighbours. And they dwell, not in a camp, but in a city far more powerful than the forces which we have brought to Sicily; [4] they are always scheming against you, and never miss a chance, as they have often shown, especially in their conduct towards the Leontines. And now they have the impudence to stir you up against those who resist them, and have thus far saved Sicily from passing under their yoke. As if you had no eyes! [5] Far more real than the security offered by them is that to which we invite you, a security which we and you gain from one another, and we beseech you not to throw it away. Reflect: the Syracusans are so numerous that with or without allies they can always find their way to you, but you will not often have the chance of defending yourself with the aid of an army like ours. And if from any suspicion you allow us to depart unsuccessful, or perhaps defeated, the time may come when you will desire to see but a fraction of that army, although, if it came, it would be too late to save you.