Pretenders are actually a sub-category of monarchic pretension. Throughout all time, there has always been the person who feels they should rule when someone, or some other entity, else is ruling. My favorite example of this is Disney's The Lion King. Scar, the king's brother, kills the king and exiles the heir apparent, thereby claiming the kingdom for himself. Simba, the exiled heir, is therefore the pretender to the throne because he is the legal heir of the last king. Scar is the usurper who legitimately holds power but did not legitimately earn the power. The Lion King, therefore, is a story of a succession crisis.
Aladdin is also a story about a succession crisis. The last male of his dynasty, the Sultan, has but one lone daughter, Jasmine, through whom to pass on the throne. Jasmine will never be a ruling queen, the succession situation in that country demands a male monarch, thus her husband will become the next sultan upon the current Sultan's death. The problem, of course, is that Jafar usurps the throne immediately upon "marrying" Jasmine, leaving the old Sultan an enslaved pretender. Yet another Disney movie about a succession crisis (also see Snow White, Cinderella, Sleeping Beauty, the Sword in the Stone, the Black Cauldron, etc.).
The common theme throughout all these films is the concept of pretension and usurpation. One person has the throne, another person wants it, and they do whatever is necessary to take it and try to legitimize it. This has happened throughout history and it isn't always as clean-cut as at the cinema. There are many different forms of pretension and they need to be examined:
- Pretender — The most common form of pretension is the pretender. A pretender can be pretty much anyone, although it usually is a disinherited heir. There are literally dozens of reasons why a pretender can appear, and anyone can be a pretender. All you need to do is so you are the monarch of someplace that already has a monarch, and get some important people to back your claim. if the public likes you enough, they may just overthrow the old monarch and make you official. Usually, though, you will die a miserable end.
The best known example of a pretender is James III of England & Scotland. In 1688, the Glorious Revolution overthrew James II from the throne, and placed instead his daughter and her husband, Mary II and William III. James II wasn't dead, mind you, and he had a son. They fled to France and over the next hundred years continued to claim the English & Scottish thrones for the Jacobites (the followers of James) despite the fact that some other people were given the throne by the people of Great Britain.
Another famous case are the Carlists of Spain, who claimed that a brother should inherit before a daughter (they also had strong political opinions). The Carlists claimed the throne for nearly a hundred years before one of their own became king as Juan Carlos I (the current king of Spain). Even so, there are still groups of Carlists who name one of his cousins the pretender, more due to politics than senior descent.
- Titular Monarch — A phenomena of especially the 1900s was the concept of titular monarchs, instead of pretenders. There is in fact a big difference between a pretender and a titular monarch that is often overlooked. While anyone can be a pretender, the main feature is that someone else has claimed the title, thereby forcing the pretension. To be a titular monarch, you need to have the entire monarchy ripped out from beneath your feet. Only non-existent monarchies can have a titular monarch. Therefore, most titular monarchs legitimately descend and would be the legitimate heir of a previous monarch that once held the title in fact, not just in name.
There is a great example for this one: the Dalai Lama. The Dalai Lama was the spiritual and temporal monarch of Tibet back in the 1950s and before. Tibet was never truly independent, but few empires pushed their will on them until the Chinese finally got around to it after the Cultural Revolution. When China invaded, the Dalai Lama fled and the theocratic state of Tibet became a Chinese province. Ever since that flight, the Dalai Lama has only been a titular monarch, although he remains a religious leader using his same title.
A much broader example is the destruction of the Central European monarchies in 1918. In the final days of World War I, German spontaneously combusted into numerous republics overthrowing their eight-hundred year-old or more monarchies. When the Treaty of Paris and its tag-along treaties were signed, pretty much all the Central Powers monarchies were disassembled including Austria, the German Empire, and the Ottoman Empire. What was left was dozens of disenfranchised titular monarchs who even today roam Europe seeking prestige from their former titles.
- Usurpers — As was mentioned at the beginning, usurpers are a type of monarch who claims the throne illegitimately from a legitimate monarch. While that leaves the legitimate monarch a pretender, it also makes the usurper a kind of pretender with power. Usurpers are actually not all that common in Europe. In most cases, the legitimate heir inherits the throne, despite the usual conception of things. Usurpers usually claim the throne when the heir is unclear. Still, while a usurper holds legitimate power, they must establish a dynasty for history to consider them a legitimate monarch and not a successful pretender.
Take William the Conqueror for example. He conquered (a term almost always synonymous with usurpation) England from the Anglo-Saxons despite the fact that they had legitimate heirs (not Harold II Godwinsson, but Edmund II Ironside's children). Yet William was able to establish his hold, "convince" the important nobles of his legitimacy, and establish a dynasty of his own. That last part is important because without it, William's fate may have been the same as the earlier Danish kings of England who, after four kings, returned the kingdom to the Anglo-Saxons.
Usurpers can also come in rather logical forms such as Catherine the Great of Russia. She was rather power-hungry and had married Peter III, the legitimate emperor of Russia. They had a son, Paul, who was the legitimate heir. But when Peter died of not-so-mysterious circumstances very soon after Paul's birth, Catherine decided that she didn't want to play regent for her infant son and instead claimed the throne for herself. She had no legitimate right to the throne, but ruled it anyway.
- Claimant — Claimants are precisely what they sound like. They claim the throne and that is basically it. Generally, claimants have a legitimate right to the throne, usually by blood, but the laws of succession are unclear leaving doubt on who should legitimately be king. This is where Braveheart comes in.
In 1290, Margaret, the so-called Maid of Norway and heiress to the throne of Scotland, died on her voyage from Norway to Britain. She was already a rather distant and abstract heir to the throne and not universally accepted. She would have been the first queen regnant in Britain had she survived the voyage. What happened instead prompted 16 years of almost continuous warfare with England. Fourteen different candidates proposed themselves as candidates to the throne of Scotland in 1290. Edward I of England, despite his rather distant claim, took the throne by force twice during this period. John Balliol, the senior-most descendant of the senior-most line of the last Scottish dynasty claimed the title successfully in 1292 but lost his throne again in 1296 when Edward and he disagreed. Enter William Wallace.
Wallace considered Balliol a traitor, since he worked with Edward of England, and decided to take the Scottish independence movement on the offensive. Despite Hollywood's portrayal of Wallace and Robert the Bruce, the two didn't get along super well. Wallace thought Bruce was a traitor, just as he had thought of Balliol, only he liked Bruce's fighting spirit and vision. When Wallace was executed, Bruce took over the fight and eventually claimed the throne successfully from Edward of England and was crowned king in 1306. He claimed the throne through Tanistry (senior kin) and proximity of blood (closest blood relative of the last king).