When Elizabeth I of England died in 1603, James VI of Scotland ascended to the English throne (as James I). Both countries then had the same monarch and leader, but were still legally separate states. An attempt to formally unite them in 1610 failed to reach an agreement.
In 1657, Oliver Cromwell created a Commonwealth of England, Ireland and Scotland, with Irish and Scottish MPs sitting at Westminster. However, this arrangement ended when Charles II was restored to the throne.
The union was finally brought about by two events. First, the failure of the Darien scheme, a Scottish colony in modern-day Panama that ruined many families who’d invested in it. Second, Queen Anne wanted to forestall efforts by supporters of James II’s son to decide who would succeed her on the Scottish throne.
By 1705, representatives from Westminster and the Scottish parliament had begun negotiations. In return for agreeing to the union and accepting George of Hanover as Anne’s successor, England gave Scotland £398,000 in debt relief – £54.8m in today’s money and equal to 80% of the Darien losses. Legislation approving the union was passed by the English parliament in January 1707 and by the Scottish parliament in March and went into effect in May.
Despite pro-Stuart uprisings in 1715 and 1745, the union remained secure. But the discovery of North Sea oil in the 1970s led to revived calls for Scottish independence. The 2014 independence referendum resulted in a “no” vote, but many see a formal separation as increasingly likely.