In August 1978, Alexander Achenbach, who describes himself as the Prime Minister of Sealand, hired several German and Dutch mercenaries to spearhead an attack on Sealand while [King] Bates and his wife were in England. They stormed the platform with speedboats, jet skis and helicopters, and took Bates' son Michael hostage. Michael was able to retake Sealand and capture Achenbach and the mercenaries using weapons stashed on the platform. Achenbach, a German lawyer who held a Sealand passport, was charged with treason against Sealand and was held unless he paid DM 75,000 (more than US$35,000 or £23,000). The governments of the Netherlands, Austria and Germany petitioned the British government for his release, but the United Kingdom disavowed his imprisonment, citing the 1968 court decision. Germany then sent a diplomat from its London embassy to Sealand to negotiate for Achenbach's release. Roy Bates relented after several weeks of negotiations and subsequently claimed that the diplomat's visit constituted de facto recognition of Sealand by Germany.
Source: Micronations, by John Ryan, George Dunford, and Simon Sellars.
EDIT: I suppose he declares himself "Prime Minister", so my title is slightly inaccurate.
Image of the prisoners taken by Prince Michael.
https://www.sealandgov.org/wp-content/uploads/2016/09/Sealand-Prisoners.jpg