Charles became King at the death of his father in the March of 1625, but was not crowned until the February of 1626.  In the summer of 1625 he married Princess Henrietta Maria, daughter of Henry IV of France and his wife Maria of Medici. As she was a Roman Catholic, the match was not popular amongst Charles's Protestant subjects. Many of his opponents believed that as he had a Roman Catholic wife, he was a secret Roman Catholic himself, and as such would be a threat to their Protestant Church. Charles and Henrietta were very happy together, however, and had a number of children, including the future kings Charles II and James II. 

While Charles was one of England’s most moral of kings (he was a devoted family man who did not take a mistress as many of his predecessors and successors did) he was one of the least popular or successful to ever sit on the throne.  Over the course of his reign, a rift grew between him and Parliament, a rift that eventually lead the country into a bloody Civil War (1642-1651). On the King’s side were the Royalists, known as Cavaliers, and on the opposite side were the Parliamentarians, known as Roundheads.  For eleven years the King had ruled without Parliament in what is known as The Personal Rule (1629-1640), and while it was not uncommon for there to be long periods inbetween parliamentary sessions, the attitude toward monarchy was not what it had been in the Tudor period. Since the Reformation, many aristocratic families had been enriched by church lands and church silver, and they were eager to consolidate their wealth with political power. In short, many believed that the monarchy had become tyrannical, and they wanted a more democratic form of government. 

For several years the Cavaliers and Roundheads battled on fields all over the country. Charles gave a good fight for his throne and showed tremendous fortitude and courage in the face of aggressive opposition, but despite a valiant effort to suppress his enemies,  the Parliamentarians eventually won. Charles was captured, found guilty of treason against his own country, and beheaded on 30 January 1649. For the next eleven years, Britain was, for the first and only time in modern history, a republic (called The Commonwealth). During these troublesome years, the country was ruled by Oliver Cromwell and then his son, Richard Cromwell, in what was effectively a military rule. Cromwell and his supporters were Puritans (ultra left-wing Protestants) who not only closed all the theaters in London (believing them to be immoral) but also banned Christmas. To some people, Oliver Cromwell is a hero, to others a tyrant who ruled by military force. Most people in  England were very glad when the monarchy was restored in 1660 and Charles I's son, also named Charles, became King Charles II.