The king being put to death, it was necessary that the Parliament should immediately determine what sort of government should succeed. Had they been disposed to continue the monarchy, and receive the eldest son of Charles, it was still necessary to take efficient means for obtaining from him, before admitting him to the throne, a recognition of all the rights for which they had striven with his father. The very day, therefore, of the king's execution, the House of Commons passed an Act, making it high treason for any one to proclaim the Prince of Wales, or any other person, king or chief magistrate of England or Ireland, without consent of Parliament; and copies of this were immediately despatched to all the sheriffs, to be proclaimed in the counties. That done, they proceeded gradually, but promptly, to develop and complete their design of adopting a Republican form of government.

The first step was to deal with the Lords. That body, or the miserable remnant thereof, still sat in the Upper House, and sent repeated messages to the Commons, to which they deigned no reply. The Lords, in fact, had become contemptible in the eyes of the whole community. They had sunk and trembled before the genius of the Commons. Though strongly inclined to stand by royalty, and though all their interests were bound up with it, though they had been created by royal fiat, and made all that they were by it, in honour, power, and estate, and though it required no great sagacity to perceive that they must fall with it, the king himself having repeatedly assured them that such would be the case, they had neither the policy nor the gratitude to hold together and maintain the fountain of their honour, nor the prescience to perceive their case when the Crown must fall, and make a merit of going over bodily to the conquering power. They had gone to pieces, some holding with one side, some with the other, some vacillating between both, changing and rechanging as the balance turned one way or the other. What was still worse, they had discovered no talent whatever on either side, with most rare exceptions, and these not remarkable, even where they had adopted a side and become partisans. Essex, Warwick, Holland, Hamilton, Newcastle, Northumberland, Ormond, and the rest, what had they done? Fairfax and Montrose, out of the whole body—and Montrose had personally been raised to it—had alone won great names. Fairfax, indeed, independent of Cromwell's hand and head, was respectable, but nothing more. The whole peerage had sunk into contemptible eclipse before the bold and vigorous genius of the Commoners. Without, therefore, deigning to answer their messages, on the 5th of February they began to discuss the question as to their retention or abolition, and the next day they voted, by a majority of forty-four to twenty-nine, that "the House of Peers in Parliament was useless and dangerous, and ought to be abolished; that the privilege of peers, of being freed from arrest, should be declared null and void, but that they might be elected knights or burgesses for the Commons." Henry Marten moved that the word "dangerous" should be omitted, and the word "useless" only be retained; or if the word "dangerous" were retained, it should be only with "not" before it, for the peers were certainly not dangerous, but pitiably useless, and they had now come to see verified what Holles had told them, that if they would not heartily join in saving the nation, it would be saved without them. An Act to this effect was soon after brought in and passed.

On the day following (the 7th), the Commons proceeded to a more important question, and voted that it had been found by experience that the office of a king in this nation, and that to have the power thereof in any single person, was unnecessary, burdensome, and dangerous to the liberty, safety, and public interest of the nation, and therefore that it should be utterly abolished; and to that purpose an Act should be forthwith prepared. This was speedily followed by a vote, on the motion of Henry Marten, that the king's statues at the Royal Exchange and other places should be taken down, and on the places where they stood should be inscribed, "Exit Tyrannus, Regum ultimus, Anno Libertatis Angliæ restitutæ primo, A.D. 1648, January 30" (old style). There was, moreover, an elaborate declaration drawn up, to justify the changing of England into a Republic, translated into Latin, French, and Dutch, and addressed to foreign States. The custody of the new Great Seal was entrusted to three lawyers—namely, Whitelock, Keble, and Lisle; they were to hold it during good behaviour, and to be called Keepers of the Liberties of England, by authority of Parliament. The King's Bench was henceforth named the Upper Bench, and came to be called the Commons Bench, and Oliver St. John, who had done so much to bring about this revolution, was made Chief Justice.

The next great measure was to dissolve the Executive Council, which had sat at Derby House, and revive it in a more extended form as the Executive Council of State, to consist of forty-one members. Three-fourths of these had seats in the House, and several of the late peers—Mulgrave, Pembroke, Denbigh, Fairfax, Lisle, Grey of Groby, Salisbury, and Grey of Werke. The chief heads of the law and officers of the army were included. The principal names were, the late peers already mentioned, and Whitelock, St. John, Cromwell, Skippon, Hazelrig, Midmay, Vane, Marten, Bradshaw, Ludlow, and Colonel Hutchinson, Governor of Nottingham. Milton, the great national poet, was appointed its secretary, and henceforth prepared its public acts, and employed his mighty talents in the defence of the measures of the Republican Government.

It was necessary to have an oath, and one was constructed which approved of the king's trial, of the vote against the Scots and their English associates, and of the abolition of monarchy and the House of Lords. But as this would not only exclude all conscientious Presbyterians, but called on the Lords to pass an act of censure on themselves, as well as on all to approve of Acts of Parliament in which they had no concern, Fairfax and some others refused to take it, and it had to be reduced to the undertaking "to be true and faithful to the Government established without king or House of Peers, and never to consent to their re-admission." This was called the "Engagement," and still was effective in excluding all Royalists, and such of the Presbyterian party as would not consent to violate their favourite Covenant. Of the twelve judges, ten had been appointed by the revolutionary party, and the whole of them had quietly continued their functions through the war against the king; yet six of these now resigned, probably having hoped to the last for an accommodation with the king, and not going in their minds the length of a commonwealth. The other six consented to hold their offices only on the condition that an Act of the Commons should guarantee the non-abolition of the fundamental laws of the kingdom.

With regard to the Church, as the present Government was decidedly in favour of ample toleration, it satisfied itself with making a slight modification of the existing Presbyterian power, and allowing it to remain, at the same time that it deprived its intolerant clergy of all temporal power whatever. No holders of religious opinions were to be molested, provided that they did not attack the fundamental principles of Christianity, and thus the Roman Catholics acquired more civil as well as religious liberty than they had enjoyed since the days of Queen Mary.

The army remained in the same able hands which had made it the finest army in Europe, and had won with it such wonderful victories. Fairfax still continued commander-in-chief, though he had held aloof from the king's trial, and the navy was put on a more efficient footing by removing the Earl of Warwick and appointing Blake, who had shown remarkable skill and courage on land, with Popham and Dean as admirals. These great changes were chiefly effected by the influence of Cromwell, Ireton, Marten, and Bradshaw, assisted by the talents of Vane, and the legal ability of St. John and Whitelock. They also introduced a Parliamentary measure, which essentially modified the character of the House. On the 1st of February they carried a vote that those who, on the 5th of December, assented to the vote that "the king's concessions were a sufficient ground to proceed to a settlement," should be incapable of sitting, but all others who should previously enter on the journal their dissent from that motion should be admissible. By this means they found the number of members raised to one hundred and fifty, and at the same time they were protected from a wearying opposition from the Presbyterian section.

They now proceeded to bring to trial such of the Royalist prisoners as had engaged in the last insurrection, whom they regarded as disturbers of the kingdom after it had once conquered the king, and might have proceeded to a settlement. They looked on them, in fact, as a species of rebels to the party in power. And yet that party was not constituted, even by its own formal enactments, as a fully recognised Government, till these trials were over. They terminated on the 6th of March, and the Republic was not formally passed till the 19th of that month, in these words: "Be it declared and enacted by this present Parliament, and by the authority of the same, that the people of England, and of all the dominions and territories thereunto belonging, are and shall be, and are hereby constituted, made, established, and confirmed to be, a Commonwealth or Free State; and shall from henceforth be governed as a commonwealth and free state, by the supreme authority of this nation, the representatives of the people in Parliament, and by such as they shall appoint and constitute officers and ministers under them for the good of the people, and without any king or House of Lords."

Whilst this Act was preparing, the trials were going on: the votes for the sitting of the Council and the Commons were considered sufficient authority. The trials were probably hastened by the news that Charles II. had been proclaimed in Scotland, and that the Scots were raising an army to avenge the king's death, and "to punish the sectaries of England for the breach of the Covenant." The persons whom it was resolved to try, were the Duke of Hamilton, the Earl of Holland, Lord Goring, lately created Earl of Norwich, Lord Capel, and Sir John Owen. The High Court appointed to try these prisoners consisted of fifty persons of both ex-Peers and Commons. The Duke of Hamilton pleaded that he was not within the jurisdiction of an English court, that he was a subject of Scotland, and a prisoner of war; but it was replied that he was also an English peer, as Earl of Cambridge, and it was proved that not only was his father naturalised as an English peer, but he himself had been called to sit as such, and had sat. The Earl of Holland was ill, and therefore made little defence, except pleading that he had free quarter given him when he was taken at St. Neots; but this was fully disproved. Lord Goring, or, as now called, the Earl of Norwich, had been a steady partisan of the king's, and had shown little lenity to the Parliamentarians; but he now conducted himself with great respect to the court, and seemed to leave himself in their hands. Lord Capel was one of the bravest and proudest of the Royalist generals. During his imprisonment he escaped from the Tower, but was betrayed by the boatmen with whom he crossed the Thames. He had expressed great indignation at the deaths of Lisle and Lucas, and had excited the resentment of Ireton by it. He now demanded to be tried by court martial, and declared that when Lisle and Lucas were adjudged to die, Fairfax had declared that all other lives should be spared, and had evidence to prove it, if he were allowed. Ireton, who really seems to have felt a stern resentment against the free-speaking general, denied that Fairfax had given any such promise, and that if he had, he had no right to supersede the authority of Parliament. He demanded that Fairfax should be sent for; but the court satisfied itself with sending to the general, who returned by letter a rather equivocating answer, saying that his promise only applied to a court martial, and not to any such court as Parliament might see fit to appoint. Bradshaw told Capel, who was not satisfied with this, that he was tried by such judges as Parliament thought proper to give him, and who had judged a better man than himself.