Spain officially entered the war on May 8, 1779 as an ally of the United States and France. The Governor of Spanish Louisiana, Bernardo de Gálvez. On August 27, 1779, Spanish forces set out to capture Baton Rouge.
On September 7, 1779, Spanish forces overwhelmed a small British garrison protecting Fort Bute, giving the Spanish control of the lower Mississippi River. The Spanish had been concerned that Britain planned to use West Florida to stage an invasion to capture New Orleans.
Three days later on September 10, the Continental Navy schooner USS Morris under the command of William Pickles defeated the aptly named British frigate HMS West Florida which was patrolling Lake Pontchartrain. The single-ship battle on each side resulted in eight deaths but more importantly weakened British control over the defenses of West Florida.
Baton Rouge & Natchez fall
From September 12-21, Gálvez attacked the British defending Baton Rouge. With Fort Bute vulnerable, a new fort Fort New Richmond was constructed hastily beginning in July. The British were eventually overwhelmed after a valiant defense. Gálvez forced the British to give up Fort Panmure in Natchez as well, leaving the entire Mississippi in Spanish or American hands, and effectively ceding the strategic defensive positions in West Florida.
In January 1780, a fleet of 12 Spanish ships arrived in New Orleans to aid Gálvez in his efforts to clear Mobile and Pensacola from British rule. On March 2, the Spanish moved with 7,500 regulars and militia on Mobile.
The British delayed sending large scale military support from Pensacola, but eventually 3,500 troops defended Fort Charlotte and Mobile. Fort Charlotte fell on March 13 and Mobile was captured the next day.
In April, The Royal Navy sent significant reinforcements to Pensacola. Gálvez was unable to capture Pensacola that summer and in January, the British with its Native American allies attempted to recapture Mobile.
On January 7, 1781, 200 Spanish regulars defeated a force of close to 750 British troops and Native American allies and fortified Mobile afterward.
In March 1781, launched a siege against Pensacola. Gálvez had close to 20,000 troops at his disposal including about 2,500 French troops. The British defended the city with only about 2,000 troops and Native Americans.
The Spanish began a siege to the fortified town on March 9. During the siege periodic attacks of Spanish forces by British regulars or Native American allies would take place, but all were beaten back.
The British and their native American allies were defeated after two months, surrendering the town on May 8.
The entirety of West Florida fell into Spanish hands by the end of May 1781. However, East Florida continued to be secure and a militarized protector of the British possessions in the Caribbean which were facing potential French attack during the final phase of the Revolutionary War.