As an active officer his military career was short, spanning barely nine years from inexperienced captain in August 1642 to all-conquering Lord General in September 1651. Probably only at Dunbar and Worcester did he have overall command in battle of armies over 10,000-strong and only in summer 1650 did he become parliamentary commander-in-chief. Nonetheless, he was the most consistently successful and conspicuously dynamic general on either side during the civil wars, a natural military genius, and his achievements, though limited to Britain and Ireland, often lead to comparisons with Marlborough, Wellington or Montgomery.
Cromwell the man of God is harder to grasp, for many of his beliefs seem far removed from our day and in some areas our knowledge is very incomplete. After his conversion, Cromwell believed that he had been chosen by God for a special duty and that thereafter God guided and generally favoured him. That did not make him a puritan kill-joy of popular image—he valued recreation, smoked a pipe, took ale, enjoyed occasional lavish banquets and appreciated singing, dancing, music and art. But it did lead him both to avoid personal sin and to encourage the extirpation of sin as part of a wider programme. During the 1640s and 1650s Cromwell came to believe that the biblical story of the Israelites was being replayed, with God freeing his chosen people, the English, from the Egyptian bondage of Stuart tyranny, leading them through the blood red sea of civil war and into the wilderness of post-war uncertainty; perhaps his role was to show the people that they were impure, convince them to reform and so propel them from the wilderness towards the Garden of Eden, winning God's love and making England a second Canaan. Thus godly reformation, the purging of sins and the creation of a more godly nation, was also a means to an end. In the early 1650s Cromwell may have believed that Christ's second coming was imminent, though this millenarianism—a belief that Christ was about to return and rule on earth for a thousand years—seems to have cooled thereafter. Cromwell also had an intense belief in divine providences, with God actively intervening in the world to shape events. This led him to interpret successes as gifts from God and signs of His favour to His worthless servant, Oliver Cromwell, but failures as rebukes from the Lord and warnings that he had strayed or sinned.
While belief in millenarianism and God's providences was common amongst the godly, Cromwell's advocacy of liberty of conscience was more unusual and distinctive. His belief that each of the new Protestant groups of the period contained an element of Godt truth led him to cherish Protestant plurality in the war and post-war years and passionately to advocate liberty for each group to worship and thrive, though he hoped that in time they might all naturally coalesce to reveal a complete mosaic of God's message. Disliking distinctive names, labels or denominations, he prayed for cooperation, congruity and mutual acceptance, 'to see union and right understanding between the godly people (Scots, English, Jews, Gentiles, Presbyterians, Independents, Anabaptists, and all).' However, Cromwell's liberty did not extend to all. In theory Catholics and rigid episcopalians were excluded—though in practice those who were loyal to the regime and discreet in their worship were not harassed during the Protectorate—as were those whose faith strayed into blasphemy and heresy, who challenged the civil authorities or who benefited from such liberty only to attack other sects. Cromwell repeatedly bewailed the failure of sects to work peacefully together, bitterly condemning those who 'put their fingers upon their brethren's consciences, to pinch them there,' who make 'wounds in a man's side and would desire nothing more than to be groping and grovelling with his fingers in those wounds. They will be making wounds, and rending and tearing, and making them wider than they are.'
In part, Cromwell the politician and statesman was moulded by his background and his secular ideas. Never a social revolutionary he supported and praised the existing order—he referred to property as one of 'the badges of the kingdom of Christ' and told parliament 'A nobleman, a gentleman, a yeoman? That is a good interest of the nation and a great one'—and he strongly opposed any group apparently agitating for social overturning or military insubordination. In some ways this conservatism was reinforced by his limited intellectual outlook, for he was not a profound or original thinker—one of his earliest biographers commented that 'It is obvious to all, he studied Men more than Books'—and down to the 1640s he had limited experience of administration and politics. During the last decade or more of his life, he was sometimes guided by, or reacted to, the initiatives of others with wider experience and sharper ideas. But far more important, Cromwell's approach to politics and statesmanship was shaped by his faith and his interpretation of God's will. In his words and actions of the 1640s and 1650s Cromwell stressed that what mattered were the ends not the means and forms of government. A regime must be seen to have God's support and be working towards God's goals, chiefly godly reformation and liberty of conscience. Once Cromwell came to believe that a regime did not have God's support or was not advancing God's cause, he not only withdrew his support but also was willing to use his power to remodel or remove it. He proclaimed that he was not 'wedded and glued to forms of Government' and that all mortal governments were but 'dross and dung in comparison of Christ.'
Thus Cromwell was willing to experiment with or change secular governments in order to advance God's work. He negotiated with the king in 1647 and sought a monarchical settlement, only to support regicide and the abolition of monarchy in 1648-49, in the process acquiescing in the drastic remodelling of the Long Parliament. He then supported the Rump, only to turn against it and destroy it in spring 1653. He experimented with a Nominated Assembly, only to accept its resignation in December 1653 and subsequently condemn it as a failure. Thereafter he became head of state and presided over the Protectoral regime in the hope this would advance God's cause. Consequently, Cromwell appears profoundly inconsistent, at times stressing the sovereignty of the people and their elected parliaments, at others snarling that the important thing was the people's 'good, not what pleases them,' at times stressing the sanctity of the law, at others commenting that 'if nothing should be done but what is according to law, the throat of the nation may be cut, till we send for some to make a law,' at times urging caution, telling his army colleagues they must 'consider the way' as well as the end, at others intervening impulsively and with little forward planning. His political path also led him to disappoint, break with or make enemies of successive waves of allies or potential allies—Presbyterians and conservative parliamentarians by his treatment of parliament, king and monarchy in winter 1648-49, Levellers and other radical groups by his increasing opposition to their agenda in the late 1640s and early 1650s, fervent millenarian groups and their allies by dropping them and supporting more secular forms of government in the 1650s, and republicans by his ejection of the Rump, the establishment of the Protectorate and the restoration of a single head of state during 1653.