Why did Oliver Cromwell end up in Ireland in the first place?

The outbreak of the English civil wars between royalists and parliamentarians in 1642 led to the execution of the English king, Charles I, on January 30th 1649. Horrified by this, hundreds of English royalists fled to Ireland to regroup. The English parliament was now encircled by threats from Scotland and Ireland. Both declared their loyalty to Charles Stuart (Charles I's son), while England became a republic and appointed Cromwell as lord lieutenant of Ireland. 

In Ireland, Cromwell faced an army comprising Irish and Old English Catholics (most of whom took part in the confederate wars of the 1640s), Irish Protestants outraged by Charles I's execution (some of whom fought against the Irish confederates) and English royalists (who fought with and against Irish soldiers in England in the 1640s). These royalist forces were commanded by the future first duke of Ormond, James Butler, whose family built Kilkenny Castle. Thus, Cromwell's time in Ireland was not a simple Irish versus English affair.

This uneasy royalist alliance controlled most parts of the country with the notable exception of Dublin. After months of preparation, Cromwell arrived in the capital with a well-equipped army. He promised to continue the 'great work against the barbarous and bloodthirsty Irish', a reference to the alleged massacres of Protestant settlers by Catholics during the 1641 rebellion. Astutely, Cromwell encouraged farmers and merchants to resume trade and warned his soldiers that he would punish those who harmed civilians. This coaxed locals to provide extra resources for, and to cooperate with, his army.

Ormond hoped to commit Cromwell's forces to a lengthy siege in Drogheda. It began on September 3rd 1649. Cromwell ordered the garrison to surrender, but his royalist counterpart Sir Arthur Aston refused. A week later, Cromwell's soldiers gained entry into the town where they met with stout resistance. Consequently, Cromwell ordered his soldiers 'in the heat of the action’ not to 'spare any that were in arms in the town’.

Despite laying their arms down (presumably in the hopes of being taken as prisoners of war), the garrison at Drogheda, comprising English and Irish Catholic and Protestant soldiers, were executed in cold blood, a breach of the contemporary rules of war. Whether civilians were killed is the subject of much historical controversy.

Cromwell wrote about the summary execution of people (it is not specified whether they were soldiers or civilians) and Catholic clergy in St Peter's Church. A civilian account suggests that troops deliberately attacked non-combatants hiding in their homes and a contemporary pamphlet noted 'many inhabitants’ were killed. News of the violent sacking of Drogheda prompted nearby garrisons (Dundalk, Carlingford, Newry and Trim) to surrender without a fight.

Cromwell subsequently headed south where he successfully took Wexford on October 11th. His troops gained entry to the town as the defenders were divided on the terms of settlement with Cromwell. One commander, Captain Strafford, opened the gates to Cromwell's troops. Allegedly, they then slaughtered 2,000 soldiers and civilians in the town.

Wexford’s fall was detrimental to the royalist war effort who were now deprived of a principal port for supplies. The killing of civilians during this siege further undermined Cromwell’s reputation in Ireland, but his successes caused the royalist war effort in Ireland to buckle as serious divisions emerged. English soldiers and Irish Protestants now began to defect to the New Model Army. 

By the end of January 1650, Cromwell controlled almost the entire eastern, northern and southern coasts (with the exception of Waterford). Soon after, he captured Fethard and Cashel. By March, despite spirited resistance from the defenders, Cromwell captured Kilkenny and Clonmel. The disintegration of the royalist war effort in Ireland and news that Charles Stuart had reached an agreement with the Scots meant that Cromwell now had to turn his attention to Scotland. 

Cromwell and his commanders were able to complete the English conquest of Ireland over the course of three years
He returned to England in May 1650 after nine months in Ireland. In June, royalist forces suffered huge losses at Scariffhollis where 2,000 troops were killed, including some high-profile veterans. Now, the royalist commander Ormond faced dwindling morale. Catholics doubted that Charles Stuart would grant them religious concessions and lost belief that the royalist war effort would triumph.

Meanwhile, Cromwell gave command of his army to his son-in-law, Henry Ireton. He set about completing the conquest of Ireland in order to redistribute the landholdings of Irish Catholics to Cromwellian soldiers and English investors in the parliamentary army of the 1640s. Thus, the name 'Cromwell' became synonymous with the actions of the New Model Army at Drogheda and Wexford and other policies imposed by the Cromwellian administration such as the transplantation of Irish Catholics to Connaught, the 1650s land settlement, and the transportation of Irish people to Barbados as indentured servants.

Buttressed by a formidable, well-funded army Cromwell and his commanders were able to complete the English conquest of Ireland over the course of three years. As one contemporary noted, this was the war that finished Ireland, 'an cogadh a chriochnaigh Éire', a historical assessment that explains his contested legacy today.