The leader of the Parliamentarian side during the English Civil War was Oliver Cromwell. Rising to prominence during the conflict, he became a key military and political figure advocating for the Parliament's cause against King Charles I. Cromwell initially served as a cavalry officer and later commanded the New Model Army, which achieved critical victories against Royalist forces. His effective leadership and strict discipline led to the eventual defeat of the king's supporters. Following the war, Cromwell played a pivotal role in the establishment of the Commonwealth of England, later becoming its Lord Protector before eventually restoring the monarchy in 1660.