Parliamentary forces refer to the military and political groups that supported the Parliament during the English Civil War (1642-1651). They opposed the Royalists, who supported King Charles I. The Parliamentary forces were primarily composed of various factions, including the Roundheads, who were known for their short haircuts and Puritan beliefs. These forces were instrumental in the eventual defeat of the Royalists, leading to the trial and execution of Charles I in 1649. The success of the Parliamentary forces helped establish a temporary republican government known as the Commonwealth of England, which lasted until the monarchy was restored in 1660.