England made several attempts to annex Scotland, but each time was eventually pushed back (such as the war of independence under Robert the Bruce). Scotland would likely have done the same had it been feasible at any point. Scotland is not particularly productive bit of landscape and is somewhat cut off from the bustling European mainland compared to England, but its harsher geography does also make it significantly easier to defend against an otherwise superior force. The various English invasions of Scotland never quite stuck.

In the 16th century England went for a different approach, looking for marriage alliances to shore up the troublesome northern border rather than expensive attempts at conquest. Scotland was allied to England's rival France and the time and England had no particular desire to be surrounded by enemies. Henry VII married one of his daughters to the Scottish King James IV, expecting this to reduce the threat of Scottish attack as well as potentially offer an avenue for England to just inherit Scotland one day. Some English nobles were concerned about bringing the Scottish House of Stewart in to the line of succession, but Henry basically brushed it off. Any potential Stewart heir would presumably be way down the line of succession since male heirs take precedence and there would have to be no surviving male descendants of Henry for the Stewart to get in via his daughter's line. Henry also figured that England would just naturally absorb Scotland anyway, writing "Should anything of the kind happen (and God avert the omen), I foresee that our realm would suffer no harm, since England would not be absorbed by Scotland, but rather Scotland by England, being the noblest head of the entire island".

Of course exactly these things that would never happen did indeed happen. Henry the VIII succeeded Henry VII as the only surviving male child. He was also the younger brother of Mary Tudor, the one that got married off to James IV. Henry VIII's male heir didn't last long, so Elizabeth I (of Civilisation V TRADE AGREEMENT WITH ENGLAND fame amongst many other accomplishments) gets the throne and proceeds to have no heirs at all. In fact, so much of the house of Tudor is gone that the next in line is actually the king of Scotland, James VI, great-grandson of Henry VII of England.

James VI of Scotland also became James I or England Ireland near the start of the 17th century.he was rather enamoured with the whole suddenly acquiring a new and vastly wealthier position thing. At this point the three crowns are still separate - held by one person, but comsidered separate nonetheless. Such personal unions are not particularly uncommon throughout history, with the Scandinavian Kalmar Union being one example. James the VI and I would not succeed in permanently uniting the crowns as he desired (he referred to himself as the king of Great Britain and Ireland, not the King of Scotland, England, and Ireland), and there was the whole thing with Oliver Cromwell interrupting the flow of monarchy for a bit, but about 100 years later his descendant Anne's reign would see the passing of the Acts of Union 1707. The Acts essentially legally abolished the crowns of England and Scotland to create a new single crown of Great Britain such that the two may never be separated. There was another act in 1800 to add Ireland in (which is the one in which they renamed it to the "United Kingdom of Great Britain and Ireland" as opposed to simply the Kingdom of Great Britain. Anne also happened to be the last Stewart monarch before the House died out and got replaced by the House of Hanover, so apparently they were running entirely on James VI and I's dying wish.

Of course, succession laws are one thing but a determined country isn't going to roll over and accept that. Scotland was overall pretty hostile to the Acts of Union, with many seeing it as accepting English domination. So why did it go through? A few things. For one, Scotland was a country of two halves - the English- or Scots-speaking Lowlands in the south and the Gaelic-speaking Highlands in the north and the islands (the latter being where you'd find the closest thing to the stereotypical image of a Scottish clansman). Starting with James IV, the Stewarts did a lot to try to de-Gaelicise the Highlands (often rather brutally). Power was stripped from the Highlands as much as possible, and many efforts were undertaken to force Highland culture to be more in line with the Lowlands, which were generally cooler with England and rapidly becoming more powerful as urbanisation set in. Secondly, Scotland was broke as fuck. This was the early days of European colonialism and Scotland wanted in on the action. Some successes in Canada were all well and good, but something big was needed to play with the major powers. The Darien scheme saw a truly absurd amount of Scotland's resources at the time (both public and private) invested in to a colony in Panama, with the intention of basically becoming the Panama Canal without the canal bit. England saw this as a serious threat and did everything possible to block it, so Scotland really was pushing its finances to the brink to fund this. Unfortunately a mixture of poor choices of goods to trade, disease, the sheer inhospitability of the land (the Darien Gap is the only break in the 30,000km Pan-American Highway from Canada to southern Argentina because of how tough it is to build things there), hostility from other European powers, Nd eventually an outright attack by Spain led to the collapse and failure of the colony. This was a colossal blow to both the Scottish economy and the Scottish national morale. Thirdly, the Acts did not entirely unite the two countries as many provisions for keeping the legal systems separate were added. Scottish law is separate from English and Welsh law to this day as a result. It includes, for example, a third trial verdict in criminal cases - guilty, not guilty, and not proven (colloquially known as "not guilty but don't do it again").

So in short, England tried invading a few times but never totally succeeded before a Scottish guy inherited both thrones and his great-granddaughter legally stuck the two together.