Capture of Fort Bute The Capture of Fort Bute signalled the opening of Spanish intervention in the American Revolutionary War on the side of France and the United States . Mustering an ad hoc army of Spanish regulars , Acadian militia , and native levies under Gilbert Antoine de St . Maxent , Bernardo de Gálvez , the Governor of Spanish Louisiana stormed and captured the small British frontier post on Bayou Manchac on September 7 , 1779 . Background . Spain officially entered the American Revolutionary War on May 8 , 1779 , 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 Gálvez , the colonial Governor of Spanish Louisiana received word of this on July 21 , he immediately began to secretly plan offensive operations . Gálvez , who had been planning for the possibility of war since April , intercepted communications from the British at Pensacola indicating that the British were planning a surprise attack on New Orleans ; he decided to launch his own attack first . To that end , he concealed from the public his receipt of the second proclamation . British defenses . Fort Bute ( ) was located on Bayou Manchac , about up the Mississippi River from New Orleans , on the far western border of British West Florida . Lieutenant Colonel Alexander Dickson was charged with the defense of the Baton Rouge district , which included Fort Bute , Baton Rouge , and Fort Panmure ( modern Natchez , Mississippi ) . The British had begun sending larger numbers of troops to the area following George Rogers Clark ' s capture of Vincennes , which had exposed the weak British defenses in the area . At Dickson ' s disposal in August 1779 were 400 regulars , including companies from the 16th and 60th Regiments and a recently - arrived company of grenadier s from the German state of Waldeck , and about 150 Loyalist militia . Fort Bute was an older stockade fort built in 1766 . It was in such disrepair that Dickson judged it to be indefensible . When Dickson received word of Spanish movements , he withdrew most of his forces to Baton Rouge and Panmure , leaving a small garrison of 20 Waldeckers under Captain von Haake behind . Spanish march . Gálvez originally planned to march from New Orleans on August 20 . However , a hurricane on August 18 swept over New Orleans , sinking most of his fleet and destroying provisions . Undeterred , Gálvez rallied the support of the colony and on August 27 set out by land toward Baton Rouge , using as an explanation for the movement the need to defend Spanish Louisiana from an expected British attack . The force departing New Orleans 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 , from Indians to 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 . When the force neared Fort Bute on September 6 , Gálvez informed them of the Spanish war declaration and the true purpose of their mission , eliciting cheers from the men . At dawn the next day , they attacked the fort , and after a brief skirmish in which one German was killed , most of the garrison surrendered . The six who escaped capture made their way to Baton Rouge to notify Dickson . Aftermath . Gálvez remained at Fort Bute for six days , giving his men time to rest , before moving on to Baton Rouge , which fell after a short siege on September 21 . The terms of capitulation agreed to by Dickson at Baton Rouge secured for Gálvez the surrender of the remaining British outposts on the Mississippi River . Gálvez then returned to New Orleans and began planning a campaign against Mobile and Pensacola , the remaining British strongholds in West Florida .