The Lelantine War was a military conflict between the two ancient Greek city states Chalcis and Eretria in Euboea which took place in the early Archaic period, between c. 710 and 650 BC. The reason for war was, according to tradition, the struggle for the fertile Lelantine Plain on the island of Euboea. Due to the economic importance of the two participating poleis, the conflict spread considerably, with many further city states joining either side, resulting in much of Greece being at war. The historian Thucydides describes the Lelantine War as exceptional, the only war in Greece between the mythical Trojan War and the Persian Wars of the early 5th century BC in which allied cities rather than single ones were involved.

Chalcis (; Ancient Greek & Katharevousa: Χαλκίς, romanized: Chalkís), also called Chalkida or Halkida (Modern Greek: Χαλκίδα, pronounced [xalˈciða]), is the chief city of the island of Euboea or Evia in Greece, situated on the Euripus Strait at its narrowest point. The name is preserved from antiquity and is derived from the Greek χαλκός (copper, bronze), though there is no trace of any mines in the area. In the Late Middle Ages, it was known as Negropont(e), an Italian name that has also been applied to the entire island of Euboea.