In the late 10th century, the Dalcassians to the south, led by Brian Boru, consolidated their hold over Munster, with Boru establishing himself as King of Munster. The ascendancy of this longtime rival kingdom posed a serious threat to High King Máel Sechnaill mac Domnaill, so the two leaders met at Clonfert in 997 and agreed upon a truce, whereby Boru was granted overlordship of the southern half of the island. The Kingdom of Leinster immediately rebelled against Boru and allied with the Norse Kingdom of Dublin. Mide and Munster formed a defensive alliance and, after a series of campaigns throughout 998–999, crushed the forces of Leinster and Dublin, which both became vassals of Munster.