The Nine Years' War, sometimes called Tyrone's Rebellion, took place in Ireland from 1593 to 1603. It was fought between an Irish alliance—led mainly by Hugh O'Neill of Tyrone and Hugh Roe O'Donnell of Tyrconnell—against English rule in Ireland, and was a response to the ongoing Tudor conquest of Ireland. The war was fought in all parts of the country, but mainly in the northern province of Ulster. The Irish alliance won some important early victories, such as the Battle of Clontibret (1595) and the Battle of the Yellow Ford (1598), but the English won a victory against the alliance and their Spanish allies in the siege of Kinsale (1601–02). The war ended with the Treaty of Mellifont (1603). Many of the defeated northern lords left Ireland to seek support for a new uprising in the Flight of the Earls (1607), never to return. This marked the end of Gaelic Ireland and led to the Plantation of Ulster. The war against O'Neill and his allies was the largest conflict fought by England in the Elizabethan era. At the height of the conflict (1600–1601) more than 18,000 soldiers were fighting in the English army in Ireland. By contrast, the English army assisting the Dutch during the Eighty Years' War was never more than 12,000 strong at any one time. Causes The Nine Years' War was caused by the clashes between the Gaelic Irish lord Hugh O'Neill and the advance of the English state in Ireland, from control over the Pale to ruling the whole island. In resisting this advance, O'Neill managed to rally other Irish septs who were dissatisfied with English government and some Catholics who opposed the spread of Protestantism in Ireland. Rise of Hugh O'Neill Hugh O'Neill came from the powerful Ó Néill sept of Tír Eoghain, which dominated the centre of the northern province of Ulster. His father, Matthew O'Neill, Baron Dungannon, was the reputed son of Conn O'Neill the Lame, the first O'Neill to be created Earl of Tyrone by the English Crown. Matthew O'Neill had been appointed by Conn as his heir, whereas Conn's eldest surviving son Shane O'Neill was the preferred heir according to the Irish custom of tanistry. After a period of warfare, Shane had Matthew murdered and became O'Neill after his father died. After the murder of Matthew's first heir, Brian, the English authorities spirited the next heir Hugh out of Tyrone to