Spain’s agreement to fund an attempted invasion of Britain in 1719 was secured through Cardinal Giulio Alberoni, a favourite of King Philip V. But Alberoni’s mail was being monitored by the Secret Department of the Post Office and the intercepted information was substantiated by other sources, including correspondence opened at Nienburg in Lower Saxony by post office staff in Whitehall’s pay.

A Spanish invasion force comprising twenty-seven vessels carrying an estimated 5,000 Spanish troops set sail from Cádiz in March 1719 with the intention of making landfall in south-west England. It was commanded by James Butler, 2nd Duke of Ormonde, who had previously served as Captain General of the British Army.

The plan was for the main invasion to be preceded by a diversionary attack in Scotland led by a contingent of Spanish troops under the command of the exiled George Keith, 10th Earl Marischal. Its aim was to twofold: to divert British troops to Scotland and, with the support of the Highland clans, to march south to join ranks with Ormonde.

In the event, a severe storm in the English Channel dictated that the Spanish invasion would fail. The main fleet was dispersed by heavy seas and those vessels that survived were instructed to return to port.

Marischal was however unaware of the disaster and continued to Scotland. His diversionary attack began with his two frigates landing three hundred troops at Loch Duich, where they overcame local resistance and captured the garrison at the mediaeval castle of Eilean Donan. But support from the clans failed to materialise and a British reconnaissance force recaptured the castle within the month. Marischal’s forces now numbered barely more than a thousand and after a few weeks skirmishing his troops were defeated on 10 June at Glen Shiel by a loyalist force. The Jacobite rebels fled and the Spanish soldiers surrendered.