[It's Christmas so, with apologies, I hope you won't mind if the following is modified from an existing essay covering this problem on my website which includes several – warning, rather gory – of the autopsy photos referred to in the text, together with an image of the peculiar "bullet-button" that is central to the discussion.]

It is certainly true that the circumstances in which Charles died were very strange.

Briefly, for those unfamiliar with the period, the king was shot through the head in December 1718, 18 years into the course of the Great Northern War, while conducting a siege at Fredrikshald, a hilltop fortress just across the Danish border in what is now Norway. The "official," and most likely explanation, is that the fatal shot was a piece of grapeshot fired from within the fortress. But there have been many who have tried to prove that the bullet or shell fragment that killed the king had not been fired from within the fortress. The "Swedish Meteor," it has been repeatedly argued, was murdered by one of his own men.

Saying with any certainty what happened to Charles XII is difficult; for one thing, while plenty of people were around him when he died, not one witnessed the instant of his death. The king had gone forward one evening after dark to supervise the construction of a front-line trench well within range of Danish musket fire. It was a deadly spot—nearly 60 Swedish trench diggers had already been killed there—and though he waited until well after dark to visit, there were flares burning on the fortress walls, and “light bombs,” an 18th-century version of star shells, illuminated the scene. Charles had just stood to survey the construction, exposing his head and shoulders above the breastworks, when he slumped forward. A large-caliber projectile had entered his head just below one temple, traveled horizontally through his brain, and exited through the far side of his skull, killing him instantly.

The first instinct of the men standing below Charles in the trench was not to investigate what had happened, but to get the king’s body out of the trenches without demoralizing the rest of the army. Later on, though, several government commissions took evidence from the men who had been in the trench that night. Most thought that the shot had come from the left–the direction of the fortress. But none had seen it strike the king.

Expert testimony makes it clear that there was nothing inherently suspicious about Charles’s death. He had been within reach of Danish guns, and might easily have been hit by grapeshot from a big gun or a sniper’s bullet. Yet there is at least a prima facie case for considering other possibilities. It has been claimed, for instance, that the guns of Fredrikshald were not firing at the time the king was hit (untrue) and that there were plenty of people on the Swedish side who might have wished Charles dead (a lot more likely). From the latter perspective, the suspects included everybody from an ordinary Swedish soldier tired of the never-ending war to the principal beneficiary of Charles’s death: his brother-in-law, who took the throne as King Frederick I, immediately abandoned the attack on Norway and soon ended the Northern War. It is possible to argue, too, that every wealthy Swede profited from Charles's demise, since one of Frederick’s first acts was to abandon a widely hated 17 percent tax on capital that the king's efficient but despised chief minister, Baron Goertz, was on the point of introducing. Goertz was so loathed by 1718 that it has been suggested that the real motive for killing Charles might have been to get to him. It is true that the baron was arraigned, tried and executed within three months of his master’s death.

The written evidence does suggest that some of those in the king’s circle behaved oddly both before and after he was shot. According to an aide-de-camp, albeit writing 35 years later, Prince Frederick seemed extremely nervous on the last day of Charles’s life and regained his composure only after being told the king was dead. And Frederick’s secretary, André Sicre, actually confessed to Charles’s murder. The value of Sicre’s “statement” remains disputed; he had fallen ill with fever, made his admission in the throes of a delirium and hastily recanted it when he recovered. But there is also an odd account that Melchior Neumann, the king’s surgeon, scribbled inside the cover of a book. The Finnish writer Carl Nordling recounts that, on April 14, 1720, Neumann

dreamed he saw the dead king on the embalming table. Then the king regained life, took Neumann’s left hand and said, “You shall be the witness to how I was shot.” Agonized, Neumann asked: “Your Majesty, graciously tell me, was Your Majesty shot from the fortress?” And the king answered: “No, Neumann, es kam einer gekrochen”—”One came creeping.”

The forensic evidence–which, perhaps surprisingly for a death that took place nearly 300 years ago, survives in abundance—offers rather surer ground. Charles’s thick felt hat, for instance, remains on display in a Swedish museum, bearing a hole 19 millimeters in diameter, or about three quarters of an inch—a clear indicator of the size, and hence perhaps the type, of the projectile that killed him. The king’s embalmed and mummified body lies in a Stockholm church, from which it has been exhumed three times–in 1746, 1859 and 1917– and on the last of these occasions X-rays were taken of the corpse and a full autopsy performed in the hope of resolving the vexed question of whether he was murdered. As we will see, even the projectile that is supposed to have killed Charles may have survived.