In late 1796, with the
French Revolutionary Wars three years old, a major shift occurred in the conflict when the
Kingdom of Spain, until that point an enemy of the
French Republic, abruptly changed sides following the signing of the
Treaty of San Ildefonso. This reversal forced the British
Royal Navy to retire from the
Mediterranean Sea to preserve its lines of communication, falling back to the mouth of the
Tagus River near
Lisbon. From the Tagus, pressure was placed on the
Spanish Navy by the erstwhile British Mediterranean Fleet under Vice-Admiral
Sir John Jervis, who organised a
blockade of the main Spanish fleet base at
Cádiz on the country's Southern Atlantic coast. This resulted in the
Battle of Cape St. Vincent on 14 February 1797, when Jervis inflicted a stinging defeat on the Spanish fleet. The surviving Spanish ships limped back to Cádiz after the battle, and to ensure that they could not sail again without risking a repeat action, Jervis arranged a close
blockade of the port so that by April there were 21 British ships of the line and numerous
frigates watching the Spanish fleet at anchor. The blockade of Cádiz was designed not only to contain the main Spanish fleet, but also to disrupt Spanish communications and transport. Cádiz was the principal port of Southern Spain and thus an important destination for shipping from across the
Spanish Empire. This included numerous so-called "treasure ships", heavily armed warships that convoyed the gold and silver from the Spanish colonies in the Americas to the Spanish mainland. ==Battle==