Ooh, this is about as perfect a question for me to answer as I'm ever likely to see, so lets hope I can do it justice!

The Solemn League and Covenant, agreed in 1643, certainly came to be seen as an agreement, imposed upon England by the Scots, to impose Presbyterianism across all Three Kingdoms.

However, its origin is slightly more complicated than that, and I think this sheds some light on how a situation arose where, seemingly, the Scots almost succeeded in setting the agenda religiously and politically for their larger neighbour in the 1640s.

The Concept of an Anglo-Scottish Covenant prior to the 1640s.

The first thing to emphasise is that, while the SLC's most direct antecedent was the 1638 National Covenant, which was specifically Scottish, the idea of some form of 'covenanted union' between England and Scotland was a concept that had been around for a while, and which had support among both English and Scottish Protestants.

While the Scottish Reformation was quite different from the English, and the forms of Protestantism that emerged from them were different, the two Reformations were still connected. Several prominent English Protestants had spent time in Scotland during the reign of Mary I, and the two Reformations became directly linked when English troops assisted Scottish Protestants in ejecting Catholic French troops from Leith in 1560.

The siege of Leith ended without significant bloodshed and is generally seen as a bit of a footnote now, but it had an enduring role in Anglo-Scottish Protestant rhetoric. It is not uncommon in the 17th century to see references to the idea that Scottish Protestants owed their English counterparts assistance, as they had saved Scotland from a Catholic yoke in 1560.

In both England and Scotland, as well, Protestants attempted to safeguard their Reformations through various forms of bonds, oaths, and covenants. In England there was the 1584 Bond of Association, and the 1606 Oath of Allegiance, both intended to guard against the prospect of a Catholic takeover. In Scotland, there was the 1580 'King's Confession' which pledged the monarch to defend Scotland's religion, and which was reprinted as the first part of the National Covenant in 1638.

The point here is that many English and Scottish Protestants viewed themselves as to some degree part of the same 'Protestant community', even though their actual religious practices were quite different (even English and Scottish Presbyterianism could vary quite a lot). And, despite these differences, Protestants in both kingdoms had started to use instruments of subscription, most commonly called Associations in England and Covenants or Confessions in Scotland, in an attempt to secure Protestantism in their respective kingdoms.

When the Scots sought to resist the imposition of the Book of Common Prayer in 1637, and, following the signing of the National Covenant in 1638, essentially seized control of Scotland and resisted the King militarily, they launched what was, for the time, a massive propaganda campaign aimed at winning over support from Protestants in England, and portraying their cause as one for Protestants in both Kingdoms.

When Charles' attempts to impose his authority by force in the 'Bishop's Wars' fizzled out and the Scots took control of Newcastle, this seeming act of divine deliverance did not just energise Scottish Covenanters, but English opposition to the King. By defeating Charles' attempts to impose the prayer book, and occupying Newcastle, in the words of Conrad Russel: 'The Scots had given power back to the English Parliament'.

Therefore, upon the outbreak of the Civil Wars in England, Scottish and English opponents of Charles I shared not only strategic goals in resisting the monarchy, but also to varying degrees shared a Protestant heritage and an appreciation of the concept of defending this heritage via bonding/covenanting. There were, therefore, people on both sides for whom the idea of an Anglo-Scottish Covenant seemed the natural next step.

Perhaps the most influential of these people was the famous English Parliamentarian John Pym, who hoped for a Covenant to unite the English and Scottish causes, and to act as a unifying shibboleth by which to determine a true and dedicated Protestant.

To be continued-