Charles I became king of England, Scotland and Ireland in 1625, inheriting kingdoms with different religious and political institutions. His marriage to the Catholic Henrietta Maria and reliance on the Duke of Buckingham heightened suspicion among Protestant opponents. Failed military expeditions and disputes over revenue brought conflict with Parliament. The Petition of Right in 1628 challenged taxation without parliamentary consent, arbitrary imprisonment and other uses of royal power. Charles accepted it while maintaining a broad understanding of his authority.

From 1629 to 1640 he governed without summoning Parliament, raising income through established prerogatives and disputed measures such as ship money. Financial expedients and religious policy accumulated opposition. Archbishop William Laud's emphasis on ceremonial worship provoked fears of Catholic influence. Attempts to impose a new prayer book in Scotland generated resistance and the Covenanting movement. The resulting Bishops' Wars exposed military and financial weakness, forcing Charles to recall Parliament in 1640.

The Long Parliament sought restraints on royal government, and the Irish rebellion of 1641 sharpened disagreement over control of armed forces. Charles's attempt to arrest five members of the Commons in January 1642 destroyed further trust. Civil war began later that year, with the king establishing his headquarters at Oxford and seeking support across his kingdoms. Royalist successes could not secure a decisive settlement. Defeat at Naseby in 1645 weakened his position, and he surrendered to the Scottish army in 1646 before being transferred to parliamentary custody in 1647.

Charles continued negotiations with different groups rather than accepting a final limitation of his authority. An agreement with Scottish supporters helped produce renewed war in 1648, strengthening the determination of army leaders to hold him responsible for further bloodshed. After opponents were excluded from Parliament, a specially created court tried him for treason in January 1649. He challenged its legal authority and refused to plead. Condemned, he was executed outside the Banqueting House in Whitehall on 30 January. His death and the temporary abolition of monarchy marked a profound constitutional rupture. His career involved disputed principles of lawful government, religious commitments and conflicts among several kingdoms. Later images of martyr or tyrant simplify the causes of his downfall.