For that we have to go back to James I of England.

As monarchs sometimes do, he had a number of children. One of these was Elizabeth Stuart. James married her off to one of the leading nobles of the Empire, Elector Fredrick V, Count Palatine of the Rhine, and eventual king of Bohemia. Sadly for Fredrick he wasn’t king for very long though dying in battle trying to defend his crown.

Fredrick and Elizabeth had 13 children themselves. Of these the 12th, Sophia, was married off to Ernest Augustus Duke of Brunswick-Lüneburg, who though war and politics was able to elevate Hanover to an Electorate.

At the same time William of Orange a Dutch noble had married Mary the daughter of James II of England and had invaded England in the Glorious Revolution, to ensure that the English throne remained in Protestant hands. However William III and Mary didn’t have any children. Rather than allow a contested succession and possibly civil war, William and parliament sought a suitable candidate. And there was Sophia, a direct descendant of James I, an impeccable Protestant, and a cousin to William himself. So in the Act of Settlement 1701 Sophia was made the heir presumptive after her cousin Anne; assuming neither William III or Anne had any children (and neither appeared to likely to).

Sophia ultimately passed before Anne did, and so never inherited. But her son George did, as George I. George was thus the king of Great Britain and Ireland while being (depending on how you count), 3/4 German, 1/8 Danish, and 1/8 British.

However the German influences didn’t end there. The next six generations of British monarchs married Germans. George I married his first cousin Sophia Dorothea of Celle. George II married Caroline of Ansbach. Frederick (who died before he could be crowned) married Augusta of Saxe-Gotha. George III married Charlotte of Mecklenburg-Strelitz. George IV married Caroline of Brunswick. His heir and brother, William IV married Adelaide of Saxe-Meiningen. His heir and daughter Victoria married Albert of Saxe-Coburg and Gotha.

It’s only when Edward VII married Alexandra of Denmark that the royal consort was a non-German. George V returned to tradition when he married Mary of Teck. But at least Mary was born in England, although she spent much of her youth on the continent. Edward VIII then abdicated to marry the American Wallis Simpson. While George VI finally married an Englishwoman in Elizabeth Bowes-Lyon. However Elizabeth then returned to form by marrying a Battenberg, although the name had been changed to Mountbatten to make it appear British.

As you might expect these lines are a bit more complicated than I’m presenting. A lot of these spouses had ancestry that went beyond Germany. But there’s no doubt the majority of the houses of Hanover, Saxe-Coburg and Gotha, and Windsor have had a decidedly German character to them.