Final answer:
Oliver Cromwell led the forces that defeated the Royalists in 1646, during the English Civil War, ultimately leading to the execution of King Charles I and the establishment of the Commonwealth of England.
Step-by-step explanation:
The man who led the forces that defeated the Royalists in 1646 was Oliver Cromwell. Cromwell was a prominent figure in the English Civil War, which lasted from 1642 to 1649. This conflict saw the forces of the monarchy, known as the Royalists, clash with Parliament's army, the New Model Army, commanded by Oliver Cromwell. The war eventually resulted in a decisive victory for the Parliamentary forces, culminating in the execution of King Charles I in 1649 and the establishment of the Commonwealth of England, with Cromwell at its head.