The most pressing motivation for Cromwell's Irish campaign was the threat of a royalist invasion. The monarchy still enjoyed great support in Ireland, and Cromwell agreed to lead an expedition into Ireland to preemptively crush all support for Charles Stuart, who remained alive in exile. Cromwell also had other motivations. He hated the Irish with a passion, largely because the vast majority of Irish people were Catholics, and he wanted to exact revenge on the Irish for a massacre of English Protestants that had occurred in Ireland 1641. Over the course of the campaign, Cromwell's eagerness to beat the Irish into submission became apparent. The two great confrontations of the Irish War were the siege of Drogheda in September 1649 and the siege of Wexford in October of the same year. Both battles ended in a frenzy of brutality, as Cromwell's army slaughtered Irish civilians and soldiers alike. At Wexford, over 2,000 people were killed inside the city after nine days of bloody resistance to the English siege. A thousand were killed in similar fashion at Drogheda after eight days of resistance. At Drogheda, Cromwell himself joined in the assault. He ordered his men to kill all priests, monks, and nuns on sight.

Until the English Civil War broke out in the summer of 1642, Oliver Cromwell was a minor figure in English politics. He received a command post early on in the war, however, and soon proved to be an exceptionally able commander. After a series of important victories, Cromwell ascended to the rank of Vice-General, and by the time of King Charles I's imprisonment and execution in early 1649, Cromwell wielded effective control of the New Model Army. His soldiers, many of whom hailed from the common classes rather than the nobility, were very loyal to him, and after Cromwell's successful campaigns in Ireland and Scotland, he became a military hero both to Parliament and the people. During the 1650s, Cromwell's control of the Army made him the most powerful man in England. He was able to dissolve the troublesome Rump Parliament and disband later parliaments that failed to pass his reforms or disagreed with him. Later, when Cromwell became Lord Protector, the military was a vital part of his rule and allowed him to enforce all of his wants and decisions.

Cromwell has a reputation for being dour, and his zealous belief in Puritan ideals did indeed give him strict views on what constituted moral behavior. With his commanding voice and personality, Cromwell was frequently forceful and assertive, but he also had a very grave temperament that bordered on the melancholic. In fact, Cromwell was known to have suffered from depression, which doubtless affected his manner and personality. In his private life, however, Cromwell had a good sense of humor and was able to enjoy a good time with friends and family.

As a leader, Cromwell was often temperamental. He was abrupt and forceful with members of the government who displeased him–at one point, for example, Cromwell forcibly dissolved the Rump Parliament when it failed to promptly enact his proposed reforms. As Lord Protector, Cromwell was equally forceful, calling Parliaments only when he felt it was necessary, and in many ways resembling the actions of the deposed King Charles I. Cromwell did, however, have a sense of restraint, and on more than one occasion refused to become King. Although he had enormous personal influence over many members of Parliament, Cromwell became increasingly isolated in his later years. Although he remained the most powerful man in England until his death in 1658, toward the end of his rule Cromwell had increasing difficulty persuading other to believe in his proposed reforms and in his religious vision for England's future.