The Battle of Baton Rouge was a brief siege during the Anglo-Spanish War that was decided on September 21, 1779. Fort New Richmond (present-day Baton Rouge, Louisiana) was the second British outpost to fall to Spanish arms during Bernardo de Galvez's march into West Florida. On May 8, Spain officially entered the Revolutionary War with a formal declaration of war by King Charles III. This declaration was followed by another on July 8 that authorized his colonial subjects to engage in hostilities against the British. When Bernardo de Galvez, the colonial Governor of Spanish Louisiana received word of this on July 21, he immediately began to plan offensive operations to take British West Florida. On August 27, Galvez set out by land toward Baton Rouge, leading a force that consisted of 520 regulars, of whom about two-thirds were recent recruits, 60 militiamen, 80 free blacks and mulattoes, and ten American volunteers headed by Oliver Pollock. As they marched upriver, the force grew by another 600 men, including Indians and Acadians. At its peak, the force numbered over 1,400, but this number was reduced due the hardships of the march by several hundred before they reached Fort Bute. On September 7, at dawn, this force attacked Fort Bute, a decaying relic of the French and Indian War that was defended by a token force. After a brief skirmish in which one German was killed, most of the garrison surrendered.

Galvez first sent a detachment of men further up the river to break communications between Baton Rouge and British sites further upriver. Before the fort he was unable to directly advance his own artillery, so Galvez ordered a feint to the north through a wooded area, sending a detachment of his poorly trained militia to create disturbances in the forest. The British turned and unleashed massed volleys at this body, but the Spanish forces, shielded by substantial foliage, suffered some casualties. This attack was eventually driven off. While this went on, Galvez dug siege trenches and established secure gunpits within musket range of the fort. He placed his artillery pieces there, opening fire on the fort on September 21. Later on the Spanish opened up with heavy artillery on the British fort. The British endured many hours of heavy mortar shelling before Dickson was offered full military surrender. When informed that Dickson had surrendered Fort Panmure, its commander was irate, believing Dickson had surrendered Panmure to get better terms of surrender. Isaac Johnson, a local justice of the peace, wrote that "In the mighty battle between Governor Galvez and Colonel Dickson, the Spaniards only lost one man and some say not one, the English lost twenty-five and the commanding officer wounded his head on his tea table". The victory at Baton Rouge cleared the Mississippi River entirely of British forces and put the lower reaches of the river firmly under Spanish control.