The battle marked an escalation in the war, as the English Crown greatly bolstered its military forces in Ireland, and many Irish lords who had been neutral joined O'Neill's alliance.

In 1598, with the besieged garrison running low on supplies, the English government debated whether to abandon the fort, as it was too far into O'Neill's home territory to be sustainable.

O'Donnell proceeded immediately, with all his warriors, both infantry and cavalry, and a strong body of forces from Connacht, to assist his ally against those who were marching upon him.

Meanwhile, O'Neill's troops had dug trenches in the countryside between Armagh and the Blackwater fort, blocked the roads with felled trees, and set up brushwood breastworks.

In Armagh, Bagenal was aware that the five miles to the besieged fort was laced with ambush positions, but believed his army could handle the hit-and-run tactics and that he would win any pitched battle.

With the main road blocked, Bagenal chose to march along a series of low hills and cross the River Callan.

[citation needed] Bagenal was the English army commander in chief (marshal) of Ulster for a decade (beginning in 1587 as his father's deputy), gaining extensive experience fighting against the Maguires and other Irish lords.

These were supported by pikemen, and targeteers; Gaelic soldiers with sword and buckler who'd been trained by Spanish advisors,[3] gave close protection to Tyrone's skirmishers.

O'Neill had several English and Spanish military advisors in his pay, as well as many Irish officers with experience in mainland Europe, who trained his troops in the use of modern weaponry.

[4] The earl had less success modernising his cavalry, who carried their spears over-arm, either thrusting or throwing them at close quarters in the traditional way.

As the lead regiment pressed on, led by Sir Richard Percy, dangerous gaps began to separate the English infantry.

Percy's regiment climbed a second hill (Drumcullen), where he found a mile-long earthen trench and bank cutting across their line of advance.

Harried by gunfire from his flanks, Percy took his regiment down the hill and over the blockade, led by the forlorn hope under captains Turner and Leigh.

This allowed O'Neill's shot to rake the compact body of troops with close-range gunfire, and his horse and swordsmen started to open gaps in the pike defence.

[8] Marshal Bagenal led his men forward to help the shattered infantry, but as he descended the hill towards the trench he was shot through the head and killed.

Two to four hundred pounds of gunpowder exploded in the English central position, killing and wounding scores and shrouding the hill in a thick cloud of smoke.

After three days of negotiation, it was agreed that the English Crown troops could leave Armagh as long as they left their arms and ammunition behind and that the garrison of the Blackwater Fort surrendered.

The most badly wounded English soldiers were left in Armagh Cathedral, many with severe burns suffered in the gunpowder explosion, but O'Neill agreed to tend to them and have them transported to Newry when they were fit to travel.