John I of England (John II, Duke of Brittany from 1286) lived 1239 - 1305 and was the first English monarch from the Capetian House of Dreux from 1274 until his death. This family were in line to the French throne and, as such, King John I behaved more as a loyal vassal to the French Kings than his royal title or his Breton ancestry would seem to imply.
Born in Brittany to a fiercely independent family that frequently played France and England off against one another, John I was an unimpressive scion of the dynasty. When it became apparent that his uncles, Theobald III and Henry II, were not going to supply the necessary amount of sons to ensure the succession, John of Brittany was groomed as a potential successor. In 1270 he accompanied St Louis IX of France and Theobald III of England on the Eighth Crusade, where he was one of the few participants to whom it wasn't apparent that Tunisia was not, in fact, part of the Holy Land of any religion. Louis' cunning plan was to outflank the Infidel, using his tiny army to push through a sweltering, disease-infested desert to the largest city in the known world, Alexandria, which had withstood several previous Crusader attacks, and, assuming that all would go well, to advance on Jerusalem, which was surrounded by dangerous enemies bored of the whole sanctimonious genocide thing. Needless to say, when Louis IX died of dysentery more or less exactly as soon as the army arrived at the first stop on its itinerary, the Crusade collapsed. John of Brittany, however, felt that the solution was clear: outflank the dangerous city of Tunis by going first to the Barbary Coast in West Africa and following the previous plan from there.
Pretty much everyone agreed that the plan was tactically impeccable, so the remains of Louis' army went with John of Brittany on a brave expedition in the opposite direction to Jerusalem and became the first Europeans to cross the Equator. Disembarking in what is now the Congo, the Crusaders marched roughly where they thought their objective was, and most died of malaria in the first three months of the trek through a jungle filled with noble savages and whatnot. A certain Friar Enguerrand of Metz recalled a nursery rhyme from his childhood about an African Christian King called Prester John and naturally came to the conclusion that he must still be around somewhere. What followed was a four-year expedition, ultimately arriving in Ethiopia in 1275. When the crusaders encountered the Emperor, Yekuno Amlak, they pronounced themselves severely disappointed and went home via Tunis, whose population had by now entirely forgotten the Eighth Crusade and were happy to share their daughters and venereal diseases with former foes. Thus, John of Brittany was one of only four survivors of the expedition.
Arriving back in Brittany in 1278, John was not recognised by anybody. Indeed, his father had forgotten he even had a son and was about to sign a will leaving all his possessions to the King of France. Over the next six years, John managed to convince his close family that he actually existed and, in 1284, his mother suddenly realised that he had been the King of England for a decade and that their wasn't a need for that sword in a stone she had been sculpting as official Regent. Exhilerated by this turn of events, John was crowned and started to rule.
After the childless deaths of both daughters of Eleanor of Aquitaine (d.1204), most of Southwestern France had been inherited by a branch of the Princes of Antioch, who were glad to escape all the war and stuff and just enjoy their retirement in the vineyards of the Dordogne. However, in 1283 the last of this line, Jean de Montfort, died of liver failure and the family died out. The closest heir was the King of Aragon, Peter III 'the Great', who immediately advanced into France with his army. At around the same time, he took advantage of the unpopularity of the new King of Naples, who happened to be the King of France's uncle, and seized Sicily. This rankled with Philippe III of France, who persuaded the Pope to declare a Crusade against his new - and newly powerful - southern neighbour. John I, veteran of two Crusades, leapt at the chance to fight for his leige lord and relative. The war lasted almost a decade, and Philippe IV, the new King of France, became increasingly aggravated by his theoretical equal's almost psychotic eagerness to please, and gave his daughter, Isabelle, to John's son and heir while also persuading him to sort out his administrative duties at home.
Ever the Breton, John I returned to Ploermel, where he founded a nunnery, laboriously made a list of all 166 of his Breton knights and felt that he had executed his duties admirably. He never visited his English domains, and simply taxed his subjects to fund the Aragones Crusade.
War of Flanders
Victorious in the Aragonese Crusade, Philippe IV felt compelled to invade Flanders to crush a rebellion. Feeling that his own lands of Champagne, Normandy and Boulogne were imperiled by the revolt, John I insisted on accompanying his master, and achieved martial credit at the Battle of Mons-en-Pévèle before the war simply petered out.
Death and Legacy
Taking a break from full-time sycophancy, John I went to Rome in 1305 to take part in the coronation of Pope Clement V. While leading the Papal mule, a balcony collapsed on him and the warrior king was crushed to death. He was buried in his nunnery at Ploermel, where even now his ghost is said to infect the Sisters with syphilis. For this, he is called a Saint by the Roman Catholic Church. Importantly, he was the first English king to act so often as a vassal of the French King, and is not regarded highly today. He was succeeded by Arthur I, his eldest son.