The Euboian League was a grouping of cities of Euboia, with Chalkis the principal one, as a political unit. The League had its own assembly and laws, though each city remained relatively independent.
One of the first recorded Greek wars was the Lelantine War between the Euboian cities of Chalkis and Eretria. Chalkis won that war, but most of the island was destroyed by the Persians during an invasion. Eretria in particular was devastated and most were taken as captives back to Persia.
After gaining their independence, Euboia then struggled with Athens until it received a lengthy independence during the Pelopennesian War. This lasted until Philip II conquered the island after the Battle of Chaeronea.
We first learn of the Euboian League during the time of Demetrios I Poliorketes. We don’t know how long it lasted, but it’s believed to have remained into Roman times. Strabo covers Euboia at the beginning of Book 10.