Charles Stuart was born in 1600 to James IV of Scotland and Anne of Denmark.  With the death of Queen Elizabeth I in 1603, James succeeded to the throne of England as King James I, and moved his family, including the three year old Charles, to England.  Upon the death of his older brother Henry in 1616, Charles became the appointed successor to the crowns of England, Ireland, and Scotland; he assumed this office following the death of his father in 1625.

From the beginning, Charles I's reign was troubled.  Most English subjects during the early Stuart period were intensely anti-Catholic, a legacy of the many dangers -- for example the Spanish Armada War in the 1580s and the Gunpowder Plot to blow up the Houses of Parliament -- the nation had experienced in earlier decades.  As king, Charles's actions ran almost completely counter to the nearly hysterical anti-Catholicism of the age:  in 1625 he married Henrietta Maria of France, a practicing Catholic, and allowed her entourage of priests to set up residence in London; in 1627, following a disastrous military campaign in France, he recalled English troops from the Thirty Years War, leaving most of western Europe prey to the military might of Catholic Spain and France.  Rather than heeding the appeals of his subjects for measures against the perceived Catholic threat, Charles I embraced a political philosophy which assumed the divine right of monarchs to form and administer national policy.  Thus, in 1629 he dismissed Parliament and proceeded to rule the nation alone for eleven years.

In the eleven years known as the "personal rule",Charles's actions continued to aggrivate his contemporaries.  In particular, his support for changes in the Church of England -- many of which seemed to place the Church's doctrine and rituals in line with Roman Catholicism -- triggered fears of a "popish plot".  When in 1640, Charles was forced to recall Parliament in order to finance a military expedition to a rebellion in Scotland, the crisis reached a head.  Radicals in the House of Commons pushed heavily for thorough-going reforms in the administration of the Church and State, and pursued virulent anti-Catholic policies.  The protests against royal policy were not limited to the Commons, particularly after news of a bloody Catholic rebellion in 1641 led many to believe that the long-feared "popish plot" against English religion and liberty was at hand.  In the midst of almost daily riots in London, Charles thus fled the capital in early 1642.

Charles I's flight from London precipitated a crisis in English politics.  The war of words between the King and his Parliament over the location of sovereignty in the nation escalated throughout 1642.  Charles claimed that the divine gift of the royal prerogative could not be steered by the whims of unruly masses; in response, Parliament claimed that the King had been seduced by evil counselors who were bent on steering the nation toward popery, and maintained that the people of the nation, embodied in Parliament, had a responsibility to rescue their monarch from the clutches of these evil interests.  The competing ideological views of kingship and authority eventually escalated into physical confrontation, with Parliamentary (or Roundhead) and Royalist (or Cavalier) armies clashing at the Battle of Edgehill in October 1642.

For the next four years, the English Civil War remained at a stalemate, with both sides unable to achieve a convincing victory.  Consequently, Parliament pursued military reforms in 1645, resulting in the formation of the New Model Army, a more efficient fighting force than that which either the Cavaliers or Roundheads had raised 1642.  These reforms shifted the tides of the war and, in May 1646, his military options rapidly disappearing, Charles I surrendured to a Scottish army at Newark, thus ending the first English Civil War.  The king was subsequently turned over the the English Parliament, and imprisoned outside London.

As negotiations for a reconciliation between the crown and Parliament proceeded in 1646 and 1647, Charles plotted to regain power without subsuming his authority to what he perceived as the tyranny of the mob in Parliament.  In late 1647, he escaped from his place of imprisonment at Hampton Court and negotiated a treaty with the Scots.  This began the second English Civil War, a conflict which saw Charles allied with English royalists and the Scots.  Unlike the first war, which lasted just under five years, this latter conflict was over within a few months.  The New Model Army, commanded by Oliver Cromwell, enacted a decisive victory over the Scots at Preston in Augsut 1648, thus forcing Charles I to again surrendur to his enemies.

By this time, Charles has gained a reputation as an "untrustworthy" king.  Although the vast majority of Parliament's supporters did not want to try the monarch as a traitor to his nation, radical elements -- particularly those in the ranks of the New Model Army -- clamored for justice.  Hence, in December 1648, the equivalent to a military coup occured in Westminster.  Those who favored negotiations with Charles were forcibly excluded from sitting in Parliament during the episode known as "Pride's Purge".  In January 1649, the handful of  MPs allowed to sit at Westminster agreed to form a High Court of Justice, convicted Charles of treason, and ordered him excecuted.  This act was carried out on 30 January, and according to an ordinance of the Rump Parliament, the crown itself was abolished.

Interpretations of Charles I's rule vary greatly.  If I have some time, I will perhaps add this information.  For now, here are some basic books and website links on the Stuart monarchy and the English Civil War:

Conrad Russell, The Causes of the English Civil War (Oxford, 199).
Lawrence Stone, The Causes of the English Revolution
Martyn Bennett, The English Civil Wars ()

Oh yeah, and a few cool pictures of Charles I sans head: