In August 1167 Henry II of England marched on Léon and captured or razed Guihomar's major castles, forcing the baron to submit and grant hostages.
Most especially he sought to protect his economic interest in the right of wreck, famously declaring that he possessed "the most valuable of precious stones," a rock which generated 100,000 solidi per annum in revenue due to shipwrecks.
[2] By 1169 Guihomar was in revolt and Henry ordered Conan IV of Brittany, who was also lord of Tréguier, which conveniently marched on Léon, to put down the disturbance.
[5] The parallels between this and the murder of Thomas Becket (1170), of which Henry was accused, probably encouraged decisive action on the king's part, so as to present himself as a defender of the assassinated churchman.
[5] In August 1177 Henry's son, Geoffrey II of Brittany, at his father's request, led a campaign against Guihomar, who was again in rebellion.