The murder of King Edward 'the Martyr' of England in 978 is the JFK assassination of its day.
Edward's rather tragic tale really kicks off with the premature death of his father Edgar in 975 when Edward is just 13. Edgar had been a powerful king and a great reformer, who had maintained an exceptionally effective defence of England and overseen a period of peace and prosperity, but who had died without naming an heir. Edward was his oldest son, but there were concerns about his legitimacy; Edgar had been married three times by his death at the age of only 32, and it's not clear which, if either, of his first two wives was Edward's mother. Edward's half-brother Æthelred was definitely legitimate, but was also a young child only some 8 or 9 years old.
Edgar's reforms had created divisions in the church between the monastic and 'secular' clergy, and had not been without opposition from some of the lesser nobility in particular. The succession crisis causes great factionalism in both the Church and the nobility, exacerbated by provincial divides. Archbishop Dunstan of Canterbury and Ealdorman Æthelwine, one of the most powerful magnates, support Edward - who is duly crowned king in 975 - but are staunchly opposed by Bishop Æthelwold of Winchester and Æthelwine's main rival, Ealdorman Ælfhere of Mercia. The situation sings perilously close to outright civil war for some time, particularly as both Æthelwine and Ælfhere use the period of instability to go on a land-grab from monastic institutions in middle England, often at the expense of those orders which had benefitted greatly from Edgar's reforms. Edward doesn't help his own cause very well, being known as a somewhat petulant child, prone to outbursts of rage and violence; according to the Vita Oswaldi, he "had offended many important persons by his intolerable violence of speech and behaviour."
In the midst of this chaos, Edward goes to meet his step-mother Ælfthryth and Æthelred at the royal residence in Corfe on the 18th of March 978, where he is promptly murdered, leaving Ælfthryth to claim the throne for Æthelred. Debate has raged for literally centuries as to the circumstances surrounding his death, and contemporary chronicles are particularly vague. Only recension 'E' notes the 'sinful' nature of his death, and it falls to 11th century manuscripts to make allegations. In his famous 'Sermon to the English' some decades later, archbishop Wulfstan of York claimed:
Edward was betrayed, and then killed, and after that burned.
Suspicion naturally fell on Ælfthryth having ordered the murder or - as Henry of Huntington later alleges - even carrying out the deed personally. The Vita Oswaldi, on the other hand, alleges that he was killed by a group of Æthelred's councillors, either in support of Æthelred, or simply as a result of a personal quarrel, noting as before that Edward was prone to losing his temper easily and flying into a violent rage. Some theories even suggest that Ælfhere of Mercia was behind the assassination, as Edward was close to reaching maturity, and Ælfhere would lose his influence, while his assassination could ruin the credibility of Æthelred and his supporters.
Just to clarify: do you mean conspiracy theories about the Middle Ages, or conspiracy theories created during the Middle Ages?
It misses the technical "middle ages" cutoff definition by 100 years or so, but the death of Amy Robsart in 1560 is a pretty good one.
Queen Elizabeth famously stayed unmarried her whole life, which was always precarious. The natural order of things expected her to marry a powerful foreigner for the purposes of political alliances and securing an heir, so there was a lot of speculation and swirl of gossip about who it might be.
Lord Robert Dudley was one of Elizabeth's favorites, despite not being a good candidate for a political alliance. He also had somewhat murky past loyalties between Elizabeth and other factions that made the match unsuitable to some, and lent an even more scandalous air to the whole thing. Worse, Dudley was already married to Amy Robsart.
Elizabeth named Dudley to the post of Master of the Horse and moved him into the palace. His wife Amy was not invited and chose to live with friends, which was a decidedly odd arrangement. Things got even more suspicious when Elizabeth subsequently turned down all other more powerful and appropriate suitors during the time that Dudley was there.
In 1560, Amy died suddenly of a "fall down the stairs" in Cumnor Place, the house she had been living in, under suspicious circumstances. She had a broken neck and two blunt force injuries to her head. The public immediately suspected that Dudley had her killed in order to be available for Elizabeth.
An official inquest was formed, which exonerated him. But the whole thing was cloaked in such a murky veil of scandal and conspiracy that, whether or not Dudley did it, Elizabeth now absolutely couldn't marry him anyway.
The official coroner's report from 1560, which was rediscovered in 2008, detailed injuries that were compatible with accident, violence, or suicide. So we still technically don't know.
My story is more about forgery so I'm not sure if it really counts as a conspiracy theory, but its definitely interesting!
I'll start this tale in the year 751. The Lombards are making a mess of Italy and have claimed swaths of land all throughout the region threatening the power and safety of the Roman Church. The current pope at the time, Pope Zachary I, was in no position to fend off the so called barbarians on his own accord. But out of sheer coincidence an opportunity in the neighboring Frankish Kingdom was a prime contender for the Pope's way out of his sticky situation in Italy.
For you see, although the Pope had no real military might to defend his city, Pepin III of the Franks had more than enough power to swoop in to Italy and oust the Lombard supremacy in the area. But of course for this quid pro quo to be effective, Pepin required something of his own. He sought to depose the current king of the Franks, the Merovingian King Childeric III, and claim the powers that came with the royal throne of the Franks for himself. But unlike Childeric, who had the "right" to hold the throne, Pepin would be seen as a usurper. Even though, as "Mayor of the Palace", he had held the real power in the background just as his lineage of Carolingians had done for decades before. Pepin yearned to be seen as the rightful king of the Frankish Kingdom and to accomplish this he sent a letter to Pope Zachary basically stating that it wasnt right for the figure who actually held the power in the kingdom not to be called king. Pope Zachary sent a letter back to him and agreed with his reasoning in a response that was recorded in the Royal Frankish Annals in 749:
Pope Zachary (r. 741-52) instructed Pepin III (r. 741-68) that it was better that the one who has the power be called king than the one who was without it. So that the order of things would not be disrupted, Zachary commaned by apostalic authority that Pepin be made king.
By this decree, Pepin was suddenly launched from de facto status to that of de jure with all the legitimacy that the royal title brought. And thus the alliance between the papacy and the Carolingian monarchy was born. And now as an ally to the Roman Church and with much to owe them, Pepin went to work as its protector by launching a campaign against the Lombard kingdom of Italy.
By the time Pepin accomplished the subjugation of the Lombards, Pope Zachary had died and was subsequently replaced by Pope Stephen II. After Pepin's accomplishment of halting the Lombard threat in Italy, the new pope travelled to the Kingdom of the Franks and in an even greater show of alliance and legitimacy anointed Pepin's young sons as the rightful heirs to the throne. But Stephen had of course travelled with other goals in mind, this is where the forgery/conspiracy takes place.
Stephen travelled all the way to the Frankish kingdom, the first Pope to ever do so, in a bid to claim the lands that Pepin had snatched back from the Lombards in Italy. The lands had historically been held by the Eastern Roman Empire as the Exarchate of Ravena, but the Pope sought ownership on behalf of the Roman Church. And to do this he used the forgery of a document which he claimed was derived from Constantine I from the 4th century. The forged document essentially claimed that by thr decree of Constantine the lands of the Western Empire, or what used to be the Western Empire before the fall, were to be under the control of the Papacy.
The forged grant purports to confer upon Saints Peter and Paul the imperial insignia, lands in Judea, Greece, Asia, Thrace, Africa, and Italy, and various islands to be disposed of at the hands of Pope Sylvester and his successors. It likewise grants that pope and his successors the city of Rome and all the province, places, and cities of Italy and the west.^1
In a bid to maintain this new unique alliance between the papacy and the Carolongian monarchy, Pepin gives in to the supposed claims that the document perpetrated, and grants the Pope his request. This act was then known as the Donation of Pepin. He allows the conquered territory to be held by the Pope and the Roman Church in turn. But truly the document went further than that. It alleges that the Pope was the overseer of the west and by giving credence to the document meant that the papacy was essentially thrust into the realm of temporal affairs.
The story presented in the Constitutum Constantini is not about events in the early fourth century, but instead about a major shift in the territorial and political position of the popes in the eighth century, over four hundred years after Constantine.^2
Throughout the greater Medieval period the document was treated as legitimate and was held up and used by the papacy as a justification of the wielding of temporal power and influence. But it goes without saying that the ruse could not go on forever, and even in as far back as the 10th century voices of suspicion rose up against the legitimacy of the document. It wasn't until the 15th century that figures worked and researched in order to provide enough proof to firmly bring the exact forgery of the document to light
Nicholas of Cusa (1401-1464) took on the task of examining the writings of the Church Fathers in order to establish whether there was any evidence for the Donation. The results of these investigations were published in his De concordanza catholica, which he finished in late 1433. Nicholas writes in this treatise that he has found no testimony for the Donation of Constantine in council records, the writings of the Church Fathers, or in papal correspondence. These observations led him to the conclusion that the document was apocryphal.^3
In 1440, seven years after Nicholas of Cusa's treatise, Lorenzo Valla composed his De falso credita et ementita Constantini donatione...and on the basis of analysing and comparing certain arguments, modern scholars have tended to assume that if Valla did not know the entire treatise, he was at least aware of the main points made by Nicholas of Cusa. Apart from employing historical arguments, as Nicholas of Cusa had done, Valla also made use of philological evidence and his antiquarian knowledge of customs and objects in late antiquity. In addition to his criticism of the form and content of the Donation, Valla stressed that it had no juridical value: even though it was incorporated in Gratian's Decretum, it was merely a palea, a later addition, which in Valla's eyes was void of any authority. He deduced from all this material that the Donation should be condemned as a forgery.^3
And there we have it! Born out of necessity and coincidence one of the most infamous forgeries of the Medieval ages was used as a ploy to secure papal suzerainty throughout the latin west. Now, whether the document was highly used is a topic of debate, especially since accusations of the document being false were prevalent pretty early on in the period after the forgery was introduced. But it is clear that the document was intended to set the stage for the papacy as a wielder of not only spiritual but also temporal power.