After the Romans withdrew from Britannia in around AD 410, the Britons organized themselves into small kingdoms. They needed to protect themselves from invaders such as the Picts from beyond Hadrian's Wall in the north, the Scots from Ireland and Germanic tribes from the European continent. King Arthur may have been a leader of one these kingdoms during this period, although he is more likely to have been a legendary figure. The peoples who later invaded and settled in Britain started to form new kingdoms of their own, for example, the Angles in Mercia and East Anglia. But some ancient British kingdoms, such as Strathclyde, survived. By 650, Britain had become a patchwork of kingdoms. A few, including Northumbria, Mercia and Wessex, became dominant, by means of warfare or through marriages between ruling families. To resist the invading Vikings in the 9th century, however, the British kingdoms were forced to unite. 

Scottish monarchs

Before the 9th century AD, Scotland was occupied by five different peoples. The Picts lived in to the north of the rivers Forth and Clyde. The Scots, who had arrived from Ireland in the fifth and sixth centuries, ruled Dalriada in the west. The Angles occupied Lothian, the ancient Britons held Strathclyde, and, in the ninth century, the Vikings started to settle in Orkney, Shetland, the Western Isles and the far north. The unification of these different peoples began in the mid-ninth century, when Kenneth MacAlpin became king of both Picts and Scots.

English monarchs

By the early 600s, the invading Anglo-Saxons had established seven main independent kingdoms in England. The seven—Kent, Sussex, Wessex in the south, Essex, East Anglia, Mercia in the midlands, and Northumbria in the north—continually fought each other for power. Northumbria dominated at first, then Mercia. After the death in 796 of the Mercian king Offa (who had married his daughters to the kings of Wessex and Northumbria and had established supremacy over England south of the Humber), Wessex became the most important kingdom and gave a united England its first kings.

Union of the Crowns

Conflict between Scottish and English monarchs persisted over the centuries, and deepened in the late 13th century, the time of the Scottish Wars of Independence, until the beginning of the 17th century. It was through the Stewart dynasty that the two thrones of England and Scotland—and later their governments—came to be united. A “Marriage of the Thistle and the Rose” (after the symbols of Scotland and England) took place at Stirling Castle in 1503 between King James IV of Scotland and Princess Margaret Tudor, daughter of King Henry VII of England. This union of the Scottish and English royal families eventually led to the the Union of the Crowns in 1603 when a Stewart (or Stuart) succeeded to the throne of England. He was King James VI of Scotland, who became James I of England.