Kingdom of England
High Middle Ages

In 1092, William II led an invasion of Strathclyde, a Celtic kingdom in what is now southwest Scotland and Cumbria. In doing so, he annexed what is now the county of Cumbria to England. In 1124, Henry I ceded what is now southeast Scotland (called Lothian) to the Kingdom of Scotland, in return for the King of Scotland's loyalty. This final cession established what would become the traditional borders of England which have remained largely unchanged since then (except for occasional and temporary changes). This area of land had previously been a part of the Anglian Kingdom of Northumbria. Lothian contained what later became the Scottish capital, Edinburgh. This arrangement was later finalized in 1237 by the Treaty of York.

The Anglo-Norman invasion of Ireland took place during the late 12th century, when Anglo-Normans gradually conquered and acquired large swathes of land from the Irish, over which the Kingdom of England then claimed sovereignty, all allegedly sanctioned by the Papal bull Laudabiliter. At the time, Gaelic Ireland was made up of several kingdoms, with a High King claiming lordship over most of the other kings.

The Duchy of Aquitaine came into personal union with the Kingdom of England upon the accession of Henry II, who had married Eleanor, Duchess of Aquitaine. The Kingdom of England and the Duchy of Normandy remained in personal union until John Lackland, Henry II's son and fourth-generation descendant of William I, lost the continental possessions of the Duchy to Philip II of France in 1204 and decisively after the Battle of Bouvines in 1214. A few remnants of Normandy, including the Channel Islands, remained in John's possession, together with most of the Duchy of Aquitaine.
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