Oliver Cromwell
The obscure country gentleman who beat a king in battle, signed his death warrant, and then ruled England himself as a military dictator in all but name.
Oliver Cromwell is the only commoner ever to rule England as head of state, and the only English head of state to be dug up and hanged. He was forty when the political crisis that made him began, a minor Huntingdon landowner with a modest farm, a seat in Parliament nobody noticed, and a conviction that God took a personal interest in his career. Twelve years later he governed three kingdoms.
The soldier
Cromwell had no military training. He turned out to be a natural cavalry commander, and more importantly a natural organizer of cavalry: his troopers were recruited for conviction rather than rank, drilled to charge and then re-form instead of scattering after plunder, and paid. That model became the New Model Army of 1645, the first professional standing army in English history, promoted on merit and stuffed with religious radicals who believed they were doing God's work. At Naseby in June 1645 it destroyed the main Royalist field army and effectively ended the first civil war. The war as a whole was a catastrophe out of proportion to its fame: something like one in twenty of the English population died, a higher share than in the First World War.
The regicide and the Protectorate
Victory raised the question nobody had planned for. Charles I, defeated, kept negotiating in bad faith and provoked a second war, and the army lost patience. In December 1648 soldiers purged Parliament of members who wanted to keep dealing with him, and the remnant put the king on trial for treason against his own people. Cromwell signed the death warrant. Charles was beheaded outside his own banqueting hall on January 30, 1649, an act with no precedent in Europe: not a murdered king, but a king tried and executed in the name of the nation.
The republic that followed could not find a constitution it liked. Cromwell dissolved the Rump Parliament by force in 1653, telling the members they had sat too long for any good they had been doing. By the end of that year he was Lord Protector, ruling with a written constitution, a standing army, and eventually a system of regional major-generals who policed morality and taxed Royalists. Offered the crown in 1657, he refused it, and kept powers greater than Charles had ever exercised. He also readmitted Jews to England after 350 years, tolerated most Protestant sects, and used the Navy to seize Jamaica and fight the Dutch for trade.
Ireland
In Ireland the memory is not ambiguous. Cromwell landed in August 1649 to crush a Catholic and Royalist alliance, and stormed Drogheda and Wexford, killing the garrisons and thousands of townspeople. He described the slaughter as God's judgment on the Irish, and it worked as terror: town after town surrendered. What followed mattered more than the sieges. The Cromwellian settlement confiscated Catholic land wholesale and pushed owners west of the Shannon, the origin of the phrase 'to Hell or to Connacht.' Catholics had held roughly three-fifths of Irish land before the war; after it they held under a tenth. Between fighting, famine, and plague, historians put Ireland's population loss in these years at somewhere between a fifth and two-fifths, one of the worst demographic catastrophes in early modern Europe.
Cromwell died of natural causes in September 1658. His son Richard lasted eight months, the army fell out with itself, and in 1660 Parliament invited Charles II home. The new regime exhumed Cromwell's corpse, hanged it at Tyburn, and put his head on a spike above Westminster Hall, where it stayed for a generation. Yet the settlement he had fought for outlasted the revenge: no English monarch after him ever governed without Parliament for long, and his statue now stands outside its doors. His career is the original template for a pattern later revolutions would repeat, from Paris to Petrograd. The men who kill a king in the name of liberty often discover that the vacancy is filled by whoever commands the army.