These tensions were increased by Spanish resentment at British control of Gibraltar and Menorca, which were confirmed by the Treaty of Utrecht. In the Anglo-Spanish War of 1727 to 1729, Spain laid siege to Gibraltar, while Britain blockaded Portobello; both attempts failed and the two countries made peace in the Treaty of Seville but the underlying issues for the conflict remained unresolved. British merchants wanted easier access to lucrative Spanish markets in the Caribbean Basin, where demand from colonists had created a large black market.