[4] 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.

[5] In resisting this advance, O'Neill managed to rally other Irish septs who were dissatisfied with the English government and some Catholics who opposed the spread of Protestantism in Ireland.

From 1591, O'Donnell, on O'Neill's behalf, had been in contact with Philip II of Spain, appealing for military aid against their common enemy and citing also their shared Catholicism.

With the aid of Spain, O'Neill could arm and feed over 8,000 men, unprecedented for a Gaelic lord, and leaving him well prepared to resist English incursions into Ulster.

[10] By the early 1590s, the north of Ireland was attracting the attention of Lord Deputy Fitzwilliam, who had been charged with bringing the area under crown control.

A provincial presidency was proposed; the candidate for office was Henry Bagenal, an English colonist settled in Newry, who would seek to impose the authority of the crown through sheriffs to be appointed by the Dublin government.

By land there were only two viable points of entry to the province for troops marching from the south: at Newry in the east, and Sligo in the west – the terrain in between was largely mountains, woodland, bog, and marshes.

Sligo Castle was held by the O'Connor sept, but suffered constant threat from the O'Donnells; the route from Newry into the heart of Ulster ran through several easily defended passes and could only be maintained in wartime with a punishing sacrifice by the Crown of men and money.

Historians disagree on the exact beginning of the Nine Years' War,[12][13] but it is generally considered to have begun in May 1593 with Gaelic lord Hugh Maguire resisting the occupation of Fermanagh.

After Willis was expelled from Fermanagh, Maguire, with the aid of MacBaron, launched punishing raids into northern Connacht, burning villages around Ballymote Castle.

In response, the crown forces were gathered under the command of Sir Henry Bagenal, who launched an expedition into Monaghan, then Fermanagh, to crush Maguire and his allies, receiving his commission on 11 September 1593.

The victory prompted uprisings all over the country, with the assistance of mercenaries in O'Neill's pay and contingents from Ulster, and it is at this point that the war developed in its full force.

The Munster Plantation, the colonisation of the province with English settlers, was dealt a serious blow; the colonists, among them Edmund Spenser, fled for their lives.

Hugh O'Neill, unable to take walled towns, made repeated overtures to inhabitants of the Pale to join his rebellion, appealing to their Catholicism and to their alienation from the Dublin government and the provincial administrations.

He spent almost all of his time in Ireland awaiting transport that he had been promised before setting out, it being the only effective way of reaching his stated objective of Lough Foyle; however, a lack of administrative efficiency in England caused his plans to go awry and the requisite pack animals and ships were never sent.

When he did turn to Ulster, Essex entered a parley with O'Neill and agreed a truce that was heavily criticised by his enemies in London, despite Elizabeth's admission soon afterward that it was "so seasonably made...as great good...has grown by it.

He was succeeded in Ireland by Lord Mountjoy, who proved to be a far more able commander, though his greater success could just as well have been because he was provided with all of the administrative support Essex lacked.

These called for a self-governing Ireland with restitution of confiscated lands and churches, freedom of movement, and a strong Roman Catholic identity.

[33] Mountjoy managed to penetrate the interior of Ulster by seaborne landings at Derry (then belonging to County Coleraine) under Henry Docwra and Carrickfergus under Arthur Chichester.

Dowcra and Chichester, helped by Niall Garve O'Donnell, a rival of Hugh Roe, devastated the countryside in an effort to provoke a famine and killed the civilian population at random.

In 1601, the long promised Spanish finally arrived in the form of 3,500 soldiers at Kinsale, Cork, virtually the southern tip of Ireland.

[38] Both he and Hugh O'Neill were reduced to guerrilla tactics, fighting in small bands, as Mountjoy, Dowcra, Chichester, and Niall Garbh O'Donnell swept the countryside.

The English scorched earth tactics were especially harsh on the civilian population, who died in great numbers both from direct targeting and from famine.

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.

Although the war had effectively ended with the signing of the Treaty of Mellifont, its final battles were fought during the English invasion of West Breifne in April 1603, which remained the sole holdout Irish kingdom following O'Neill's capitulation.

The kingdom was ruled by Brian Óg O'Rourke, one of the alliance's chief lieutenants and leader of the Irish forces during the Battle of Curlew Pass.

He failed to secure any concessions from the treaty as his half-brother Tadhg O'Rourke had fought with the English during the war and was granted lordship of West Breifne in return.

Following a twelve-day siege, a force of 3,000 men led by Tadhg, Henry Folliott, and Rory O'Donnell eventually brought the area, and thus all of Ireland, under English control on 25 April 1603.

O'Neill, O'Donnell, and the other Gaelic lords from Ulster allied to them left Ireland in 1607 in what is known as the "Flight of the Earls" after news they planned another rebellion reached the authorities.

They intended to organise an expedition from a Catholic power in Europe, preferably Spain, to restart the war but were unable to find any military backers.