As Alastair Bellany and Thomas Cogswell explain in the greatest possible detail, James, an old man long in indifferent health, almost certainly died of natural causes, as official sources claimed at the time. It seems equally certain that his favorite, the Duke of Buckingham, made an irregular if well-intentioned intervention in his treatment, sending the King a plaster and a potion, but there is no better evidence that this proved fatal. The political and diplomatic context, however, was propitious to other readings. James's death in 1625 occurred in a court riven by personal rivalries, and a nation that harbored ideological divisions, intensified by foreign policy. The old King had been committed to keeping his realms out of war, but shortly before his death Buckingham, and James's own heir Charles, had turned vehemently to an opposed course, of conflict with Spain. James's demise could thereby be deemed to have occurred at a convenient moment for both of them, enabling Charles to open his own reign by attacking the Spanish, while retaining Buckingham as his best friend, adviser and executive agent.Nor did the accusation fade away quickly. It was revived in the Civil War period, when it was extended to the King himself, and in fact it remained a staple of Puritan historiography down to the period of the American revolution.
Rumors that Buckingham's medical intervention had been disastrous therefore blended with darker suspicions that it might have been maliciously intended. In 1626, these were employed in an attempt to discredit the war policy, by a Scottish Catholic exile in the pay of Spain, who wrote a pamphlet, smuggled into England and distributed, which accused Buckingham of having poisoned not only James but a leading nobleman, the Duke of Hamilton, who had likewise become inconvenient to him. This convinced some private commentators, and seems to have been influential in persuading the House of Commons to include an accusation of having intervened fatally (if not necessarily deliberately) in the late King's treatment, in the impeachment that it launched against Buckingham soon after. Charles's insistence of standing by his favorite led to the failure of the Parliament concerned, which in turn contributed to the King's decision to rule without any Parliaments through the 1630s.
Historians know about these conspiracy theories and disagree about them in the same way modern pundits disagree about our fake news: a few think they are important, even decisive, but until recently the majority thought that they were believed only by people already predisposed to hate the accused poisoners. The point of Bellany and Cogswell's book, The Murder of King James I, is to argue that this calumny was historically important. Ronald Hutton reviewed the book in the September 16 TLS, and he offers this summary of the changing historical consensus:
Indeed, their book takes its place among a succession of works that have further eroded the nineteenth-century portrait of the Civil War and Revolution as the inevitable outcome of profound social change and of the rational self-interest of a national spirit rising up against outmoded and oppressive ideologies. It makes an important addition to a growing sense of the political actors of the time as people propelled by largely questionable fears, extending from the cosmic (anxiety about personal damnation or divine wrath against their whole society) to the transitory (a terror of specific conspiracies by ideological enemies to destroy all that those imagining them held dear in their world).