The significant political figure during the English Civil War was Oliver Cromwell. As a military leader and statesman, Cromwell played a crucial role in the conflict between the monarchy and Parliament. He led the Roundheads, the parliamentary faction, to victory against King Charles I, which ultimately resulted in the king's execution and the establishment of the Commonwealth. Cromwell's leadership and reforms had a lasting impact on English governance, promoting republican ideals, although his later rule as Lord Protector raised questions about governance and power.