In 1643, English Parliamentary forces were in retreat. The King seemed to be on the verge of triumph in the English civil war. However the war involved not merely the English Parliament but also the Scottish Kirk. In 1638-40 the Scots had rebelled, the Irish rebelled in 1641 and the royal forces were attempting to deal with a problem across the three kingdoms not merely in the England. In 1643 the English Parliament went to negotiate with the Scots about them joining in the war. The treaty which brought the Scots in was called the Solemn League and Covenant. The Solemn League and Covenant is a fascinating document because, as Colin Kidd argues in a recent set of lectures on the issue of unionism in Scotland, it provides a template for understanding Scottish and English unionism.
The Solemn League and Covenant was not merely an alliance against Charles I in the English Civil War: it was also a constructive document that envisaged a world after the civil war. Because it was negotiated at a period in which the Scots were powerful, the Solemn League and Covenant bound the English to fulfill Scottish conditions. It envisaged a partial political union- an eternal alliance between the English Parliament and the Scottish one- but did not envisage the marriage either of the two countries' politics or of their laws. Rather the document envisaged a union of the two nations' religions: England and Scotland would be united according to the Solemn League and Covenant in Presbyterian faith and Church government would proceed according to the best Calvinistic model. Both Bishops (the Anglican form of church government) and independent congregations (the type of government favoured by independents like Oliver Cromwell) would be abolished or restricted. Though the Solemn League and Covenant was never fully enforced and was abandoned as the balance of power changed in England- it did have some lasting effects including an assembly of Scottish and English divines and scholars at Westminster which debated theology for five years and produced a confession of faith for the whole nation.
The Solemn League and Covenant provides an important template for thinking about Scottish and English union in the 16th and 17th Century. The imagined reality of two Presbyterian nations, perpetually Protestant and perpetually allied, facing a Catholic Europe was not one that suddenly emerged in 1643. One can see a similar idea (if backed by a different ecclesiology) in William Cecil's plans for the destruction of Marian Scotland in the 1560s or in John Knox's plans for the evangelisation of the English court in the 1540s. Part of the reason that both in England and Scotland union was popular in the Early Modern period was that it represented a way to unite Protestant forces against a Catholic other- whether France or Spain- that threatened to use Britannic minorities to overthrow the religious settlements in both nations. Perhaps one of the interesting reflections that that provokes is that national identity in the early modern period may have been weaker in comparison to religious identity than it is today.