As the attack had been made in time of peace, and was perfectly unexpected, there were of course men and stock in the fields; and the Thebans wished if possible to have some prisoners to exchange against their countrymen in the town, should any chance to have been taken alive.

Such was their plan.

But the Plataeans suspected their intention almost before it was formed, and becoming alarmed for their fellow citizens outside the town, sent a herald to the Thebans, reproaching them for their unscrupulous attempt to seize their city in time of peace, and warning them against any outrage on those outside.
Should the warning be disregarded, they threatened to put to death the men they had in their hands, but added that, on the Thebans retiring from their territory, they would surrender the prisoners to their friends.

This is the Theban account of the matter, and they say that they had an oath given them.

The Plataeans, on the other hand, do not admit any promise of an immediate surrender, but make it contingent upon subsequent negotiation: the oath they deny altogether.