Battle of Halys

The Battle of Halys, also known as the Battle of the Eclipse, took place at the river Halys (now Kızılırmak, in Turkey) on May 28, 585 BC between the Medes and the Lydians. The final battle of a five-year war between Alyattes II of Lydia and Cyaxares of the Medes, the battle ended abruptly due to a total solar eclipse; the eclipse was perceived as an omen, indicating that the gods wanted the fighting to stop.

Since the exact dates of eclipses can be calculated, the Battle of the Eclipse is the earliest historical event of which the date is known with such precision. (This date is based on the proleptic Julian calendar, which does not include a "year zero"; astronomically the year is -584.) See also Mursili's eclipse and the eclipse of Bur Sagale.

Causes
Herodotus (Histories, 1.73-74) states that there were two reasons for the war; the two sides clashing interests in Anatolia, but also there was a motive of revenge. Some Scythian hunters employed by the Medes who once returned empty-handed were insulted by Cyaxares. In revenge the hunters slaughtered one of his sons and served him to the Medes. The hunters then fled to Sardis, the capital of the Lydians. When Cyaxares asked for the Scythians to be returned to him, Alyattes refused to hand them over; in response, the Medes invaded.

Aftermath
A truce was hastily arranged. As part of the terms of the agreement, Alyattes's daughter Aryenis was married to Cyaxares's son Astyages, and the river Halys was declared to be the border of the two warring nations.

The eclipse
According to Herodotus (1.74): In the sixth year a battle took place in which it happened, when the fight had begun, that suddenly the day became night. And this change of the day Thales the Milesian had foretold to the Ionians laying down as a limit this very year in which the change took place. The Lydians however and the Medes, when they saw that it had become night instead of day, ceased from their fighting and were much more eager both of them that peace should be made between them.

According to NASA, the eclipse peaked over the Atlantic Ocean at 37.9°N, -46.2°W and the umbral path reached south-western Anatolia in the evening hours, and the Halys River is just within the error margin for delta-T provided.

An alternative theory regarding the date of the battle suggests that Herodotos was recounting carelessly events that he did not witness personally and furthermore the solar eclipse story is a misinterpretation of his text. According to this view, what happened could have been a lunar eclipse right before moonrise, at dusk. If the warriors had planned their battle activities expecting a full moon as in the previous few days, it would have been quite a shock to have dusk fall suddenly as an occluded moon rose. If this theory is correct, the battle's date would be not 585 BC (date given by Pliny based on date of solar eclipse), but possibly 3 Sept 609 BC or 4 July 587 BC, dates when such dusk-time lunar eclipses did occur.