In 1780, Berkeley was appointed to HMS Fairy, a 14-gun brig under his cousin George Keppel and together they captured the American ship Mercury, taking prisoner Henry Laurens who was on a secret mission to loan money from the Dutch government. The information procured from Laurens led to a British declaration of war against the Netherlands. As another consequence, Berkeley was promoted to captain by Admiral Richard Edwards and commanded Fairy during the relief of the Great Siege of Gibraltar and further operations against American shipping from Newfoundland.
