James Oglethorpe had established Savannah and the Georgia Colony in 1733 and Fort Frederica on St. Simons Island in 1736 to protect the southern boundary of his new colony from the Spanish in Florida.
The War of Jenkins' Ear gave Oglethorpe an excuse to move against the Spanish. On June 13, 1740, Oglethorpe began the siege of St. Augustine by blockading the city including the Matanzas Inlet. Anticipating Oglethorpe's attack, Governor Manuel de Montiano had earlier sent a courier to Havana asking for supplies since there were only enough for three weeks.
It is lucky that Florida had a man like Manuel de Montiano as governor during this crucial time. The Castillo had survived the attack of 1702, but Moniano made it even stronger by raising the walls and putting in the arched casements. Now cannon could be mounted all around the gun deck. He also built the Cubo Line and other outer defense works as well as Fort Mose, a settlement of escaped African slaves from the Carolina Colony to whom Montiano granted citizenship and freedom in return for their serving in the militia.
Anticipating Oglethorpe's attack, Montiano was able to successfully send a shallow-draft vessel down the Matanzas river and out the Matanzas Inlet before the blockade had tightened to sail to Cuba with a message for supplies since there were only enough for three weeks.
Oglethorpe placed troops and cannon batteries on Anastasia Island to fire on the city and the Castillo. He hoped that a sustained bombardment and blockade of St. Augustine would cause Montiano to surrender the city and fortress to the British. The English guns fired on the Castillo, but were unable to breech the walls which were at the farthest extent of the British cannon range. In another action, the British did capture Fort Mose, but it was retaken by the Spanish.
On July 7, the courier which had been sent to Cuba returned to St. Augustine and told Montiano that six supply ships were at Mosquito Inlet, 68 miles further down the coast. (Present-day Ponce de Leon Inlet) He also told Montiano that the British had withdrawn the vessel blocking Matanzas Inlet, and the way appeared clear to provision the city. However, an English deserter reported to the Spanish that Oglethorpe planned a night attack during the next six days of unusually high tides, for the high water was needed to cross Matanzas Bay and assault the Castillo.
Six days passed and no attack came, so Montiano sent five small vessels down the Matanzas River, out the Matanzas Inlet, and on to Mosquito Inlet to fetch the supplies. Just as the boats returned to the Matanzas Inlet, they met two British sloops that were taking soundings. The sloops opened fire and took up the chase. The fighting continued until twilight when the British sloops returned to their squadron. Their withdrawal gave the Spanish flotilla the opening they needed. They promptly entered the Matanzas Inlet, sailed up the river, and safely anchored at St. Augustine.
Fearing the approaching hurricane season, the British fleet decided to sail north for safer waters. Lacking naval support and knowing that the city was now well-supplied, Oglethorpe raised the siege on July 20, 1740.