The Acts of Union 1707 took effect on 1 May 1707, finally uniting the Kingdoms of England and Scotland into one, united country – Kingdom of Great Britain.
The counties were at the point de facto united for over a century, ever since James VI of Scotland ascended to the English Throne as James I on 24 March 1603, after the death of Elizabeth I. However, back then, the two countries merely entered into a Personal Union of Crowns – a situation when two or more countries share the same Monarch, while remaining, separate, sovereign states (not unlike, say, Canada, Australia and the United Kingdom now).
There had been previous attempts at unification through the 17th century. Indeed, when James VI became King of England, he announced his intention to unite the two Kingdoms so that he wouldn’t be “guilty of bigamy”. He styled himself as King of Great Britain and declared the country should be viewed as “presently united, and as one realm and kingdom, and the subjects of both realms as one people’.