Also called O'Neill's Cavalry March, the O'Neill refers to Aodh Mór Ó Néill (Hugh O'Neill) and The Nine Years' War (Irish: Cogadh na Naoi mBliana or Cogadh Naoi mBlian) or Tyrone's Rebellion took place in Ireland from 1594 to 1603 fought between the forces of Gaelic Irish chieftains Hugh O'Neill of Tír Eoghain, Hugh Roe O'Donnell of Tír Chonaill and their allies, against English rule in Ireland. The war was fought in all parts of the country, but mainly in the northern province of Ulster. It ended in defeat for the Irish chieftains, which led to their exile in the Flight of the Earls and 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.

The Nine Years' War was caused by the collision between the ambition of the Gaelic Irish chieftain Hugh O'Neill and the advance of the English state in Ireland, from control over the Pale to ruling the whole island. The Pale (An Pháil in Irish) or the English Pale (An Pháil Sasanach), was the part of Ireland that was directly under the control of the English government in the late Middle Ages. It had been reduced by the late 15th century to an area along the east coast stretching from Dalkey, south of Dublin, to the garrison town of Dundalk. The inland boundary went to Naas and Leixlip around the Earldom of Kildare, towards Trim and north towards Kells.

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.

In the aftermath, The English Lord Mountjoy smashed the O'Neills' inauguration stone at Tullaghogue, symbolically destroying the O'Neill clan. Famine soon hit Ulster as a result of the English scorched earth strategy. O'Neill's uirithe or sub-lords (O'Hagan, O'Quinn, MacCann) began to surrender and Rory O'Donnell, Hugh Roe's brother and successor, surrendered on terms at the end of 1602. However, with a secure base in the large and dense forests of Tir Eoghain, O'Neill held out until 30 March 1603, when he surrendered on good terms to Mountjoy, signing the Treaty of Mellifont; Elizabeth I had died on 24 March.

The leaders of the rebellion received good terms from the new King of England, James I, in the hope of ensuring a final end of the draining war that had brought England close to bankruptcy. O'Neill, O'Donnell and the other surviving Ulster chiefs were granted full pardons and the return of their estates. The stipulations were that they abandon their Irish titles, their private armies, their control over their dependents and swear loyalty only to the Crown of England. In 1604, Mountjoy declared an amnesty for rebels all over the country. The reason for this apparent mildness was that the English could not afford to continue the war any longer. Elizabethan England did not have a standing army, nor could it force its Parliament to pass enough taxation to pay for long wars. Moreover, it was already involved in a war in the Spanish Netherlands. As it was, the war in Ireland (which cost over £2 million) came very close to bankrupting the English exchequer by its close in 1603.

Irish sources claimed that as many as 60,000 people had died in the Ulster famine of 1602–3 alone. This is likely to be a major overestimate, as in 1600 the total adult population of Ulster has been estimated at only 25,000 to 40,000 people.  An Irish death toll of over 100,000 is possible. At least 30,000 English soldiers died in Ireland in the Nine Years' War, mainly from disease. So the total death toll for the war was certainly at least 100,000 people, and probably more.

Although O'Neill and his allies received good terms at the end of the war, they were never trusted by the English authorities and the distrust was mutual. O'Neill, O'Donnell and the other Gaelic lords from Ulster left Ireland in 1607 in what is known as the Flight of the Earls. They intended to organize an expedition from a Catholic power in Europe to restart the war, preferably Spain, but were unable to find any military backers.