The High Court of Justice was the court established by the Rump Parliament to try Charles I, King of England, Scotland and Ireland. Even though this was an ad hoc tribunal that was specifically created for the purpose of trying the king, its name was eventually used by the government as a designation for subsequent courts.
Background
The English Civil War had been raging for nearly an entire decade. 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 English 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 war deaths totalled approximately 3.6% of the population, estimated to be around 5.1 million in 1650.
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.
Only seventy-five people were allowed to enter and, even then, only at the army's bidding. On 13 December, the "Rump Parliament", as the purged House of Commons came to be known, broke off negotiations with the King. Two days later, the Council of Officers of the New Model Army voted that the King be moved to Windsor "in order to the bringing of him speedily to justice". In the middle of December, the King was moved from Windsor to London.
The role of Parliament in ending a reign
Neither the involvement of Parliament in ending a reign nor the idea of trying a monarch was entirely novel. In two prior examples, the parliament had requested both the abdication of Edward II, and that of Richard II, in 1327 and 1399 respectively. However, in both these cases, Parliament acted at the behest of the new monarch. Parliament had established a regency council for Henry VI, although this was at the instigation of senior noblemen and Parliament claimed to be acting in the King's name.
In the case of Lady Jane Grey, Parliament rescinded her proclamation as queen. She was subsequently tried, convicted and executed for high treason, but she was not brought to trial while still a reigning monarch.
Establishing the court
After the King had been moved to London, the Rump Parliament passed a Bill setting up what was described as a High Court of Justice in order to try Charles I for high treason in the name of the people of England. The bill initially nominated 3 judges and 150 commissioners, but following opposition in the House of Lords, the judges and members of the Lords were removed. When the trial began, there were 135 commissioners who were empowered to try the King, but only 68 would ever sit in judgement. The Solicitor General John Cook was appointed prosecutor.