It was in 585 B.C. when a solar eclipse took place, during which two warring states - the Medians and the Lydians - were in the midst of battle. For these ancient people, a solar eclipse would have appeared to have been an omen from the gods. Subsequently, they lay down their weapons and quickly ushered in a truce.
The most likely date for this event was on May 28, 585 B.C., thanks to historical records and to calculations made by modern astronomy, with which we are able to pinpoint the occurences of solar eclipses in history. Other dates have been have been proposed with varied evidence, but May 28 is the most widely-accepted date for the event..
This wasn't the first solar eclipse to be recorded, but it was the first to be accurately predicted. According to the Greek historian Herodotus, Thales of Milete had made the prediction that it would occur in the year that it did.
How he made the prediction remains unknown, and the answer continues to elude modern historians, because prediction of such events requires certain astronomical knowledge that they weren't thought to have had back then.