In 1778 and early 1779, American George Rogers Clark seized British forts and trading posts at Kaskaskia, Cahokia, and Vincennes (in today’s Illinois and Indiana) in the Old Northwest with the ultimate objective of capturing Detroit. In June 1779, Spain entered the war against the British. From their trading post at St. Louis near the confluence of the Mississippi and Missouri Rivers, the Spanish supplied arms and ammunition to Clark and the Americans which prevented the British from retaking the area. The British decided to strike directly at St. Louis.

St. Louis was the administrative capital of Upper Spanish Louisiana, but it was little more than a village of 900. The lieutenant governor of Louisiana, Captain Fernando de Leyba had only 29 regulars of the Spanish Colonial Louisiana Regiment, and about 150 militia consisting of French, Spanish, and Creole citizens of the town, along some friendly Indians and African slaves. His force was not nearly enough to repel a determined assault. In March 1780 Leyba was warned by a fur trader about an attack, and learned that his superiors in New Orleans could not spare any additional troops for St. Louis’ defense. Leyba’s and St. Louis’ townspeople financed the defenses themselves. Leyba appealed to French fur trader François Vallé, who gathered another 150 French militia from those who didn’t respond to Leyba’s call to arms. Leybas and Vallé planned to build four great towers connected by a trench to protect the town, but when the British and their Indian allies arrived in late May, only the northwest tower and the trench were completed.

The plan to seize St. Louis was actually spawned in London, and confirmed that the British government had no concept of the vast distances involved on the North American frontier. British militia captain Emanuel Hesse managed to gather a force of about 1000, mostly Canadian fur traders whom he promised exclusive rights, and Indians marginalized by the Spanish. The Sioux, Menominee, Chippewa, Winnebago, Sauk, and Fox Indians had been on the receiving end of Spanish policies of arming their neighbors to fight against them. Hesse promised them revenge. They gathered in Prairie du Chien (in today’s Wisconsin) and Chippewa war chief Matchekewis took charge of the Indians. When Hesse and Matchekewis arrived outside St. Louis, they sent 300 warriors to fix Clark at Cahokia to prevent him from coming to Leyba’s aid.

When the warning shots sounded the alarm on the morning of 26 May 1780, Leyba gathered everyone he could inside of St. Louis, and his men in the trenches. The round and squat stone tower at the northwest corner of the town was the centerpiece of his defense. Dubbed “Fort San Carlos” after the king of Spain, King Charles III, the fort housed all of Leyba’s cannon. Leyba had four four pound cannon and two six pound cannon.

Hesse and Matchekewis tried rushing the town because they knew they could never hold the Indian coalition together long enough for a siege. The Indian warriors charged across the open fields opposite the trench and Fort San Carlos into the waiting Spanish guns filled with French lead from one of Vallé’s mines. The cannons of Fort San Carlos were especially effective. Although the Sauk and Fox had infrequently encountered muskets before, they had never encountered cannon. The massive eruptions terrified the Sauk and Fox and they immediately deserted Hesse. The direct assault failed, and there was no way to convince the Indians of another try. Hesse then attempted to force the Spanish to come out and fight. He had prisoners taken from the outlying farms tortured in full view of the town just outside of the range of Fort San Carlos guns. Leyba convinced his soldiers and townspeople to wait it out, and not sortie. Hesse’s force melted away that night once it was obvious they couldn’t take the town without another direct assault. George Rogers Clark defeated the Indian force sent to Cahokia the same day.

The Battle of Fort San Carlos was the only American Revolutionary War battle fought west of the Mississippi River. The British defeat ended the possibility of further British and Indian alliances in the Old Northwest for the rest of the war. With the trading post at St. Louis secure, George Rogers Clark secured the Old Northwest for the nascent United States of America. With the west bank of the Mississippi under Spanish control, and the east bank under American control, the British could not prevent the Old Northwest from being ceded to the United States in the Treaty of Paris in 1783. The Spanish victory at the Battle of Fort San Carlos ended the possibility of the Appalachian Mountains being the western border of the United States after the American Revolution.