Following the failure of the 1715 Rising, James Stuart’s supporters turned to Sweden and Spain for financial and logistical support. Alerted by agents on the continent and at home, the Secret Department of the Post Office was tasked with monitoring communications between Georg Heinrch von Görtz, Sweden’s minister plenipotentiary in Europe, and Carl Gyllenborg, the Swedish ambassador in London.

Although written in Swedish diplomatic code, a cypher based on a combination of mathematics, French and Swedish, London succeeded in decrypting the correspondence. The substance was so disquieting that regardless of international law and that it would constitute an assault on diplomatic immunity, Ambassador Gyllenborg was arrested on 29 January 1717, his residence searched, and his papers seized. Nothing was found. Gyllenborg, forewarned, had destroyed all incriminating letters and his key to the diplomatic cipher.

Sweden responded to the arrest with fury and retaliated by arresting the British ambassador in Stockholm. Britain went public to justify its actions. Gyllenborg’s intercepted correspondence was published and the government initiated an anti-Swedish propaganda campaign co-ordinated by Charles Delafaye and led by Daniel Defoe. A selection of Gyllenborg’s letters were reproduced in the press, accompanied by editorials outlining British grievances against Sweden.

Charles XII’s Sweden, a Protestant monarchy and former ally, had its defenders and a large minority in Parliament viewed the dispute as motivated by Hanoverian interests in Europe. When George I put pressure on Parliament to suspend trade with Sweden, many MPs took a contrary position. Britain and Sweden stepped back from confrontation and released their respective hostages. But Gyllenborg’s experience of arrest and public denigration reinforced his enmity. Notwithstanding that the invasion planned for 1717 was abandoned, on their return to Sweden, Gyllenborg and von Görtz began another operation to place James Stuart on the British throne. That scheme would be put into execution two years later with Sweden backing a Spanish military and naval invasion of Britain.