Kennedy was devoted to the
Presbyterian cause in Scotland against the efforts of
King Charles I to impose an
Anglican form of
church polity on the northern kingdom in 1638. In 1639, strongly sympathetic to
Covenant theology, Cassillis was among the 20,000 Covenanters who met the king's army at
Duns Law, a show of force which resulted in royal permission to summon a free
General Assembly and to seat a free
Parliament to ratify the Assembly's acts. In 1643, Cassillis represented Scotland's
Solemn League and Covenant at the
Westminster Assembly, a convention of English divines and Members of
Parliament of
Calvinist persuasion who desired to reduce episcopal influence in the Church of England and reorganize with elders, or
presbyters, in the manner of the
Scottish Church. After the defeat of the Covenanters at the
Battle of Kilsyth in 1645, Cassillis fled to
Ireland, but in the next year was one of the Scottish commissioners who met with King Charles to gain his assent to the decisions of the
English (
Puritan) Parliament. However, in 1648, he opposed the
Engagers, a Covenanter faction desirous of treating with Charles I. After the execution of Charles I in
London, in January 1649, Cassillis was among the seven commissioners who met with
Charles II (whom the Scots, unlike the English, continued to recognize) in
The Hague the following March. They hoped to persuade the young king to accept both the Scottish National Covenant of 1638 and the Solemn League and Covenant of 1643. They further insisted that he enjoin acts of the Scottish Parliament on England and Ireland. On 19 May Charles replied that he would accept the Scottish acts relating to the National Covenant and Presbyterian doctrine and would adopt anything in the Solemn League and Covenant that benefitted Scotland without injuring England or Ireland; however, he would not overthrow the recent peace agreement between England and Ireland. The commission regarded this response as a rejection of their demands and returned to Scotland, landing at
Leith on 27 May. In February 1650, Cassillis was in attendance at a meeting of the Committee of Estates, whose members were divided into two strongly opposed groups. One group wanted to send commissioners to treat with Charles at
Breda in the
Netherlands, his headquarters in exile. This group included
Argyll, Loudoun, and all the lords present except Cassillis. But rumours were about that Charles was consolidating an alliance with Irish
Catholics, which drove the other faction on the Committee of Estates to double down on the demand that Charles acknowledge the legality of the sitting Scottish Parliament. Argyll's group at first prevailed, but in the debate regarding the instructions to be given to the commissioners, Cassillis' opinion prevailed and the group was commissioned "to require of Charles the same absolute surrender which had been required of him at The Hague". To provide balance, representatives of both camps were included in the commission, with Cassillis representing the firmer position on behalf of the Covenant. Cassillis also served as one of the commissioners of the Kirk who were joining in their secular counterparts on the embassy to Breda. On 25 March the commissioners met with Charles in the Netherlands and put forward the following demands: • He was to consent to the establishment of the Presbyterian system in England and Ireland. • He was to practice Presbyterianism personally and enforce its practice amongst the members of his household. • He was to acknowledge the legality of the recent sessions of the Scottish Parliament. • He was to agree that all secular matters in Scotland would be decided by the Parliament; and ecclesiastical matters, by the general assemblies of the Kirk. • He was to suspend toleration of Catholicism and cancel all declarations that were deemed prejudicial to the Covenant. Anticipating Charles' response,
William II, Prince of Orange, offered to negotiate a compromise, but the commissioners, who had been cautioned upon their departure from Scotland not to negotiate anything beyond what their commission stated, would not relent. Aware that Charles was having difficulty solidifying support in England, Cassillis and two other commissioners, acting through the Prince of Orange, sought to put his mind at rest: If in any particular our answers be not fully satisfactory to his Majesty's desires, we humbly conceive it more expedient that his Majesty, putting himself on the affections of his [Scottish] people, should refer them to his [Scottish] Parliament where his Royal presence will obtain more than we are warranted to grant, Charles, despite a few more attempts at negotiation, assented to this proposal, though historian Samuel Rawson Gardiner has noted that behind the scenes "the
Duke of Lorraine, the
Queen of Sweden, and the Prince of Orange . . . combined in urging him to promise anything with the direct intention of breaking his word whenever he was strong enough to do it with impunity". Perhaps in this hope, the king raised his hackles once more and indicated that his willingness to execute
penal laws against the Catholics notwithstanding, he would not consent to annual reconstructions to the treaties with the Irish and would protect the Second Peace Treaty (1649) which had exempted Irish Catholics from the penal laws. Still, lacking anything more than moral support from his allies, Charles could only capitulate and, accordingly, on 28 April sent a private note to Cassillis promising "to insert the required form of words after his landing in Scotland if the Parliament should require him so to do", which Gardiner views as proof that the king was marvellously naïve in his understanding of the resolve of the Scottish lawmakers. On 1 May Charles signed a draft of his agreement with the commissioners, which is known in history as the
Treaty of Breda. Satisfied, the commission invited Charles to Scotland on 29 May. ==After the Restoration==