As the two coalitions grew, their separate interests kept coming into conflict. Under the influence of King Archidamus II (the Eurypontid king of Sparta from 476 BC through 427 BC), Sparta, in the late summer or early autumn of 446 BC, concluded the Thirty Years Peace with Athens. This treaty took effect the next winter in 445 BC Under the terms of this treaty, Greece was formally divided into two large power zones. Sparta and Athens agreed to stay within their own power zone and not to interfere in the other’s. Despite the Thirty Years Peace, it was clear that war was inevitable. As noted above, at all times during its history down to 221 BC, Sparta was a “diarchy” with two kings ruling the city-state concurrently. One line of hereditary kings was from the Eurypontid Dynasty while the other king was from the Agiad Dynasty. With the signing of the Thirty Years Peace treaty, Archidamus II felt he had successfully prevented Sparta from entering into a war with its neighbours. However, the strong war party in Sparta soon won out and in 431 BC Archidamus was forced to go to war with the Delian League. However, in 427 BC, Archidamus II died and his son, Agis II succeeded to the Eurypontid throne of Sparta.
Source: Wikipedia
