Histories (1.75.1-1.75.5)

Having this reason to quarrel with Cyrus, Croesus sent to ask the oracles if he should march against the Persians; and when a deceptive answer came he thought it to be favorable to him, and so led his army into the Persian territory.

When he came to the river Halys, he transported his army across it—by the bridges which were there then, as I maintain; but the general belief of the Greeks is that Thales of Miletus got the army across.

The story is that, as Croesus did not know how his army could pass the river (as the aforesaid bridges did not yet exist then), Thales, who was in the encampment, made the river, which flowed on the left of the army, also flow on the right, in the following way.

Starting from a point on the river upstream from the camp, he dug a deep semi-circular trench, so that the stream, turned from its ancient course, would flow in the trench to the rear of the camp and, passing it, would issue into its former bed, with the result that as soon as the river was thus divided into two, both channels could be forded.