The Parliamentarian Army was the military force that supported the Parliament during the English Civil War (1642-1651). It was primarily composed of troops loyal to the Roundheads, who opposed the royalist forces led by King Charles I. The army played a crucial role in the conflict, advocating for parliamentary sovereignty and opposing the king's absolute rule. Led by notable figures such as Oliver Cromwell, the Parliamentarian Army utilized innovative tactics and disciplined formations. Their victories in key battles, including the Battle of Naseby, helped secure the eventual defeat of the royalists, leading to the establishment of a republican government in England.