The Glamorgan Treaty, 1645

During the first year of the English Civil War, King Charles I formulated plans to recruit an army from Ireland to fight against Parliament. In September 1643, the King's Lord-Lieutenant, the Marquis of Ormond, concluded the Cessation of Arms with the Irish Confederates which allowed government troops stationed in Ireland to return to England. However, the military advantage gained by the King proved negligible and the Royalist cause suffered political damage through parliamentary propaganda which represented the returning troops as ferocious Irish papists. By 1645, Parliament's alliance with the Scottish Covenanters threatened to overwhelm the Royalists. The King desperately needed more troops in England, particularly after his crushing defeat at Naseby in June 1645, and was prepared to recruit Irish Confederates despite the potential political damage to his cause amongst English and Scottish Protestants.

The King appointed Edward Somerset, the son of the Marquis of Worcester, to negotiate with the Confederate Assembly, creating him Earl of Glamorgan and promising him a dukedom and the marriage of his eldest son to the Princess Elizabeth if the mission succeeded. Working independently of the Marquis of Ormond, Glamorgan negotiated a secret treaty with the Confederates by which Roman Catholics in Ireland were to be exempted from the jurisdiction of Protestant clergy and were granted possession of all churches they had seized since the outbreak of the Irish Uprising in 1641. In return, the Confederates were to raise an army of 10,000 men to serve the King in England. The First Glamorgan Treaty was signed on 25 August 1645.

In November 1645, the papal nuncio Archbishop Rinuccini arrived at Kilkenny and joined in the negotiations. Rinuccini demanded further concessions, to which Glamorgan readily agreed: the King was to undertake never to appoint a Protestant lord-lieutenant in Ireland, Catholic bishops were to be allowed to sit in the Irish Parliament and a Catholic university was to be established in Ireland. In return, Glamorgan was to be appointed commander of an advance guard of 3,000 Confederate soldiers to sail immediately for the relief of Chester. The so-called Second Glamorgan Treaty was signed on 20 December 1645 and Glamorgan hurried to Dublin to secure Ormond's consent to his appointment as commander of the advance guard.

Meanwhile, a copy of the first treaty had been discovered when the Archbishop of Tuam was killed by a Scottish raiding party in October 1645. The terms of the treaty shocked Protestant Royalists in Dublin, who assumed that Glamorgan had forged his commission from the King. When he arrived in Dublin, Glamorgan was brought before Ormond and charged with treason by Lord Digby. Ormond made a theatrical denial of any knowledge of Glamorgan's activities and the Earl was placed under arrest while the matter was referred to the King. On 16 January 1646, news of the treaty reached the Westminster Parliament, leading to demands for the King's deposition. King Charles made a public disavowal of the treaty on 29 January but he wrote privately to Ormond ordering him to suspend the proceedings against Glamorgan, who was released on bail.

Glamorgan returned to Kilkenny and resumed negotiations with Archbishop Rinuccini, to whom he swore his devotion. He supported Rinuccini's promotion of the terms negotiated in Rome by Sir Kenelm Digby, the Queen's envoy to the Pope, which included the repeal of all laws against Catholics in England as well as in Ireland. During February 1646, Glamorgan continued his preparations for the expedition to relieve the siege of Chester, but on 8 March, news arrived that the city had already fallen to the Parliamentarians. Ten days later, Glamorgan learned that King Charles had publicly disavowed him.