(Walker, History of Independency, i. 71, ed. 1661). As yet he was not prepared to abandon the monarchy, and for a time supported the plan of deposing the king and setting the Prince of Wales or Duke of York on the throne (ib. p. 107; Gardiner, Great Civil War, iii. 294, 342).
In the second civil war Ireton served under Fairfax in the campaigns in Kent and Essex. After the defeat of the royalists at Maidstone he was sent against those in Canterbury, who capitulated on his approach (8 June 1648) (Rushworth, vii. 1149; Lords' Journals, x. 320). He then joined Fairfax before Colchester, and was one of the commissioners who settled the terms of its surrender (Rushworth, vii. 1244). To Ireton's influence and to his ‘bloody and unmerciful nature’ Clarendon and royalist writers in general attribute the execution of Lucas and Lisle (Rebellion, xi. 109; Mercurius Pragmaticus, 3–10 Oct. 1648; Gardiner, Great Civil War, iii. 463). Ireton approved the decision of the council of war which sentenced them to death, and defended its justice both in an argument with Lucas himself at the time and subsequently as a witness before the high court of justice. There is no foundation for the charge that the sentence was a breach of the capitulation [see Fairfax, Thomas, third Lord Fairfax].
The fall of Colchester (28 Aug.) was followed by a renewal of agitation in the army, and Ireton's regiment was one of the first to petition for the king's trial (Rushworth, vii. 1298). Already a party in the parliament was anxious that the army should interpose to stop the treaty of Newport, but Ludlow found Ireton strongly opposed to premature action. He thought it best ‘to permit the king and the parliament to make an agreement and to wait till they had made a full discovery of their intentions, whereby the people, becoming sensible of their danger, would willingly join to oppose them’ (Ludlow, Memoirs, p. 102). About the end of September Ireton offered to lay down his commission, and desired a discharge from the army, ‘which was not agreed unto’ (Gardiner, Great Civil War, iii. 473–5). For a time he left the headquarters and retired to Windsor, where he is said to have busied himself in drawing up the army remonstrance of 16 Nov. 1648 (reprinted in Old Parl. Hist. xviii. 161). All obstacles to agreement among the officers of the army were removed by the king's rejection of their last overtures. ‘It hath pleased God,’ wrote Ireton to Colonel Hammond, ‘to dispose the hearts of your friends in the army as one man … to interpose in this treaty, yet in such wise both for matter and manner as we believe will not only refresh the bowels of the saints, but be of satisfaction to every honest member of parliament.’ He conjured Hammond, in the national interest, to prevent the king from escaping, and endeavoured to convince him that he ought to obey the army rather than the parliament (Birch, Letters to Hammond, pp. 87, 97). In conjunction with Ludlow he arranged the exclusion of obnoxious members known as ‘Pride's Purge’ (Memoirs, p. 104). In conjunction with Cromwell he gave directions for bringing the king from Hurst Castle; he sat regularly in the high court of justice, and signed the warrant for the king's execution (Nalson, Trial of Charles I, 1684).
During December 1648 the council of the army was again busy considering a scheme for the settlement of the kingdom, which resulted in the ‘Agreement of the People’ presented to the House of Commons on 20 Jan. 1649 (Old Parl. Hist. xviii. 516). The first sketch of the ‘Agreement’ was not Ireton's, but by the time it left the council of war it had been revised and amended till it substantially represented his views. While a section in the council held that the magistrate had no right to interfere with any man's religion, Ireton claimed for him a certain power of restraint and punishment. Lilburne complains that Ireton ‘showed himself an absolute king, against whose will no man must dispute’ (Legal Fundamental Liberties, 1649, 2nd ed. p. 35). Outside the council of war his influence was limited. The levellers hated him as much as they did Cromwell, and denounced both in the ‘Hunting of the Foxes by five small Beagles’ (24 March 1649) and in Lilburne's ‘Impeachment of High Treason against Oliver Cromwell and his son-in-law, Henry Ireton’ (10 Aug. 1649). With the parliament he was, as the chief author of the ‘Agreement,’ far from popular, and though he was added by them to the Derby House Committee (6 Jan. 1649) they refused to elect him to the council of state (10 Feb. 1649).
On 15 June 1649 Ireton was selected to accompany Cromwell to Ireland as second in command, and set sail from Milford Haven on 15 Aug. His division was originally intended to effect a landing in Munster, but the design was abandoned, and he disembarked at Dublin about the end of the month (Commons' Journals, vi. 234; Murphy, Cromwell in Ireland, p. 74). During Cromwell's illness in November 1649, Ireton and Michael Jones commanded an expedition which captured Inistioge and Carrick, and in February 1650 he took Ardfinnan Castle on the Suir (Carlyle, Cromwell's Letters, cxvi. cxix.).