High Court of Justice for the trial of Charles I

The High Court of Justice was the court established by the Rump Parliament to try King Charles I of England. This was an ad hoc tribunal created specifically for the purpose of trying the king, although the name was used for subsequent courts.

After the first English Civil War, the parliamentarians accepted the premise that the King, although wrong, had been able to justify his fight, and that he would still be entitled to limited powers as King under a new constitutional settlement. By provoking the second Civil War even while defeated and in captivity, Charles was held responsible for unjustifiable bloodshed. The secret "Engagement" treaty with the Scots was considered particularly unpardonable; "a more prodigious treason," said Oliver Cromwell, "than any that had been perfected before; because the former quarrel was that Englishmen might rule over one another; this to vassalize us to a foreign nation." Cromwell up to this point had supported negotiations with the king but now rejected further negotiations.

In making war against Parliament, the king had caused the deaths of thousands. Estimated deaths from the first two English civil wars has been reported as 84,830 killed with estimates of another 100,000 dying from war-related disease." The population of England in 1650 was estimated at only 5.1 million, meaning that the war deaths totalled 3.6 percent of the population.

Following the second civil war, the New Model Army and the Independents in Parliament were determined that the King should be punished, but they did not command a majority. Parliament debated whether to return the King to power and those who still supported Charles's place on the throne, mainly Presbyterians, tried once more to negotiate with him.

Furious that Parliament continued to countenance Charles as King, troops of the New Model Army marched on Parliament and purged the House of Commons in an act later known as "Pride's Purge" after the commanding officer of the operation. On Wednesday, 6 December 1648, Colonel Thomas Pride's Regiment of Foot took up position on the stairs leading to the House, while Nathaniel Rich’s Regiment of Horse provided backup. Pride himself stood at the top of the stairs. As Members of Parliament (MPs) arrived, he checked them against the list provided to him. Troops arrested 45 MPs and kept 146 out of parliament.

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