The United Kingdom is made up of four constituent countries: England, Wales, Scotland and Northern Ireland. But there have long been tensions between England and the other three U.K. states, in part because England has always been the dominant political power among them. England brought all the states together through conquest and political union. Here’s how that happened.

The Kingdom of England, formed in 927, gained the first U.K. state other than itself through invasion. In the late 13th century, King Edward I conquered the western Principality of Wales, claiming it as a territory of England. Next, he invaded the northern Kingdom of Scotland, kicking off the First War of Scottish Independence (that’s the one in Braveheart).

Scotland emerged from the First and Second Wars of Scottish Independence with its sovereignty intact. Wales, meanwhile, remained a conquered territory. Beginning with Edward I, English monarchs gave their successors the title of “Prince of Wales” to signify their control over the territory. This tradition continues today.

Still, Wales was not an official part of the Kingdom of England until the 1530s and ‘40s. Under King Henry VIII, England passed Acts of Union extending English laws and norms into Wales. This was the first major political union in what would become the U.K.

When Queen Elizabeth I died in 1603, the next person in line to the throne was her cousin, King James VI of Scotland. Now, he gained a second name: King James I of England.

Even though Scotland and England shared the same king, they were still two politically separate kingdoms, each with its own parliament. Over the next century, there were several failed attempts to merge them into one nation. These attempts ended in 1707 when England and Scotland united as “Great Britain” under Queen Anne (the queen portrayed in The Favourite).

There were several reasons for this union, says Christopher A. Whatley, a professor of Scottish history at the University of Dundee and author of The Scots and the Union: Then and Now. One was the fact that Scotland was in debt after trying to establish a colonial empire in the Americas the same way that England, Portugal and Spain had done.

“The Scots recognized that the Realpolitik, if you like, of the situation was that if they were to establish markets overseas, contacts overseas, they needed the support of a stronger maritime power, which was England,” he says.

Many Scots also saw the union as a way of preventing the Catholic Stuarts from reinstating an absolute monarchy and securing Scotland’s future under a Protestant constitutional monarchy. For England, there was concern that if it didn’t unite with Scotland, the country might side against England with France in the War of the Spanish Succession. So in 1707, England agreed to give Scotland money to pay off its debts, and both countries’ parliaments passed the Acts of Union to become one nation.

Remember how King James VI of Scotland was also King James I of England? Well, he was actually King James I of Ireland, too. Back in the 1540s, Ireland become a dependent kingdom of England, and the 1542 Crown of Ireland Act mandated that the king of England was now also the king of Ireland. The first person to hold both titles was Henry VIII. The last was George III, who oversaw the 1801 creation of the United Kingdom of Great Britain and Ireland.

Whatley says England used its 1707 union with Scotland as a model for Great Britain’s 1801 union with Ireland. However, the Irish Union didn’t last nearly as long. Between 1919 and 1921, the Irish Republican Army fought for independence from the U.K. The Irish War of Independence ended with the division of Ireland into northern and southern regions in 1922.

The northern region remained a part of the U.K., which changed its title to the United Kingdom of Great Britain and Northern Ireland. The southern region became the Irish Free State, which, despite its name, was still a part of the British Commonwealth. In 1937, the southern region became the sovereign nation of Ireland (or the Republic of Ireland). It joined the European Union in 1973 and is still a member nation today.

After World War II, there was an increase in nationalism in Wales, Scotland and Northern Ireland. This took the form of activism, violent conflict and the formation of political parties that emphasized independence from the U.K.

In December 2019, discussions about Scottish independence and Irish reunification increased after an election ensured Conservative party leader Boris Johnson would remain U.K. prime minister and continue with the plan to withdraw the United Kingdom from the European Union, as mandated by a June 2016 referendum known as Brexit. Brexit was much less popular in Scotland and Northern Ireland than in England. One way for both states to remain in the E.U. would be by declaring independence from the U.K. In Northern Ireland’s case, this could mean reuniting with the Republic of Ireland.

Scotland already held an independence referendum in 2014, in which it voted by 55 percent to remain in the U.K. But a lot has changed since then. In the wake of the December 2019 election, Scotland’s First Minister Nicola Sturgeon said she would take steps to call for another independence referendum. Whatley says, “The Union of 1707 is closer now to being broken up than any time in its history.”