Oglethorpe invested St. Augustine deploying his batteries on the island of Santa Anastasia while the naval squadron blockade the port. When Oglethorpe began a 27 day bombardment on June 24, the Spanish colonist families, some 2,000 people, all sought refuge within the fort, Castillo de San Marcos. Both sides suffered serious losses to disease with about half their troops incapacitated. On June 26, a sortie by 300 Spanish and free blacks attacked Fort Mose held by 120 Highlander Rangers and 30 Indians. The garrison was taken by surprise with 68 killed and 34 captured while the Spanish loss was 10 killed.
The Spanish managed to send supply ships through the Royal Navy blockade and any thought of starving St. Augustine into capitulation was lost. Oglethorpe now planned to storm the fortress by land while the navy ships attacked the Spanish ships and half-galleys in the harbor. Commodore Pearce, however resolved to forgo the attack during hurricane season. Oglethorpe gave up the siege and returned to Georgia abandoning his artillery.