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 intercepted by the Secret Department of the Post Office and the information revealed was substantiated by other sources, including letters intercepted at Nienburg in Lower Saxony by German post office staff in Whitehall’s pay.
A large-scale invasion force was put together in Spain and some 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. The force 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, whose fleet sailed from Spain in advance aiming to tie-down British troops in Scotland and, supported by the Highland clans, to march south to join ranks with Ormonde’s forces.
In the event a storm in the English Channel dictated that the invasion would fail. The main Spanish fleet was dispersed by heavy seas and the vessels that survived were instructed to return to port. Unable to communicate with Ormonde, Marischal was unaware of the disaster and continued to Scotland.
Marischal’s diversionary attack began with 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 the anticipated support from the clans failed to materialise and a British reconnaissance force recaptured the castle within the month. Marischal’s total forces numbered barely more than a thousand and after several weeks skirmishing his small army was defeated on 10 June at Glen Shiel by a loyalist force. The Jacobite rebels fled and the Spanish troops surrendered.