With the departure of Porsenna, war broke out between Rome and the other Latin states leading to the Battle of Lake Regillus in either 499 or 496 BC. In 493 BC a treaty, the Foedus Cassianum, was concluded, establishing a mutual military alliance between the Latin cities with Rome as the leading partner. A second people, the Hernici, joined the alliance sometime later. While the precise workings of the Latin League remains uncertain, its overall purpose seems clear. During the 5th century the Latins were threatened by invasion from the Aequi and the Volsci, as part of a larger pattern of Sabellian-speaking peoples migrating out of the Apennines and into the plains. Several peripheral Latin communities appear to have been overrun, and the ancient sources record fighting against either the Aequi, the Volsci, or both almost every year during the first half of the 5th century BC. This annual warfare would have been dominated by raids and counter-raids rather than the pitched battles described by the ancient sources.