The coastal town of Drogheda was held by a mixed garrison of Irish Catholics and Royalists under the command of Sir Arthur Aston, when it was besieged by English Commonwealth forces under Oliver Cromwell.

Some Royalist Protestants now changed sides, and James Butler, 1st Duke of Ormonde had to rally the remaining dispersed forces to put together a new field army.

[4] With troops largely composed of veterans from the New Model Army, Oliver Cromwell landed near Dublin in August 1649 to re-conquer the country on behalf of the Commonwealth of England.

[5][6][b] Ormonde's strategy was to avoid battle, while holding towns in the east of Ireland, relying on hunger and disease to weaken the besiegers.

[8] He, therefore, favoured rapid assaults on fortified places using his siege artillery, rather than time-consuming blockades to secure the all-important ports.

[13] In a letter to William Lenthall, Speaker of the English House of Commons, written shortly after the storming of the town, Cromwell explained why he did not fully invest the town, an action that would have left his divided command vulnerable to an attack by a relieving force and a simultaneous sortie by the garrison, but rather concentrated his troops on the south side of Drogheda for a swift assault.

If we had divided our force into two-quarters to have besieged the North Town and the South Town, we could not have had such a correspondency between the two parts of our Army, but that they might have chosen to have brought their Army, and have fought with which part 'of ours' they pleased,—and at the same time have made a sally with 2,000 men upon us, and have left their walls manned; they having in the Town the number hereafter specified, but some say near 4,000.The Parliamentary commander set up his batteries at two points near the Duleek gate, on either side of St Mary's church (on the site of the current Gerrard's Church), south west of, and near the Millmount Fort, where they would have an interlocking field of fire.

Sir, having brought the army of the Parliament of England before this place, to reduce it to obedience, to the end that the effusion of blood may be prevented, I thought fit to summon you to deliver the same into my hands to their use.

The surviving defenders tried to flee across the River Boyne into the northern part of the town while Aston and 250 others took refuge in Millmount Fort overlooking Drogheda's southern defences.

"[19] In Cromwell's words, "In the heat of the action, I forbade them [his soldiers] to spare any that were in arms in the town...and, that night they put to the sword about two thousand men".

[22] After breaking into the town, the Parliamentarian soldiers pursued the defenders through the streets and into private properties, sacking churches and defensible positions as they went.

[26] Some of the Royalists like Aston were Englishmen who had been taken prisoner and then released on military parole in England in 1647–48, but had carried on fighting for King Charles in Ireland.

[29] Any Catholic clergy found within the town were clubbed to death or "knocked on the head" as Cromwell put it[30] including two who were executed the following day.

Three days after the storming of the town, Sir Edmund Verney, an Englishman, was walking with Cromwell, when he was called aside by a former acquaintance who said he wished to converse with him, but instead of a friendly greeting, he was run through with a sword.

[35][d] Two days afterwards, Lieutenant-Colonel Richard Boyle, an Anglo-Irish Episcopalian, was dining when an English Parliamentary soldier entered and whispered something to him.

Hugh Peters, a military chaplain on Cromwell's council of war, gave the total loss of life as 3,552, of whom about 2,800 were soldiers, meaning that between 700 and 800 civilians were killed.

Irish clerical sources in the 1660s claimed that 4,000 civilians had died at Drogheda, denouncing the sack as "unparalleled savagery and treachery beyond any slaughterhouse".

[39] Cromwell justified his actions at Drogheda in a letter to the Speaker of the House of Commons, as follows:[40] I am persuaded that this is a righteous judgement of God on these barbarous wretches, who have imbrued their hands with so much innocent blood; and that it will tend to prevent the effusion of blood for the future, which are satisfactory grounds for such actions which cannot otherwise but work remorse and regret.Historians have interpreted the first part of this passage, "the righteous judgement of God," in two ways.

[41] Historian John Morrill has argued that, in fact, it was English Royalist officers who were singled out for the most ruthless treatment—being denied quarter, executed after being taken prisoner and whose heads were publicly displayed on pikes.

[26] Some analyses by authors such as non-historian Tom Reilly, have claimed that Cromwell's orders were not exceptionally cruel by the standards of the day, which were that a fortified town that refused an offer of surrender, and was subsequently taken by assault, was not entitled to quarter.

[45] However, others have argued that while, "Arthur Aston had refused a summons to surrender, thereby technically forfeiting the lives of the garrison in the event of a successful assault... the sheer scale of the killing [at Drogheda] was simply unprecedented".