San Juan Expedition ( 1780 ) The San Juan Expedition took place between March and November 1780 during the American War of Independence when a British force under the command of John Polson and Captain Horatio Nelson landed on the coast of the present - day Nicaragua , with the aim of sailing up the San Juan River to capture the strategically crucial towns of Granada and León , located on the northwestern shore of Lake Nicaragua . Despite an initial success in the capture of the Fortress of the Immaculate Conception , Polson ' s force never reached Lake Nicaragua and , decimated by yellow fever , was forced to return to Jamaica . The campaign ended in total failure and cost the lives of more than 2 , 500 men , making it the costliest British disaster of the entire war . Background . After Spain entered the American Revolutionary War in 1779 , Major - General John Dalling , the governor of Jamaica , proposed a military expedition against the Spanish province of Nicaragua , belonging then to the Captaincy General of Guatemala , a dependency of the Viceroyalty of New Spain . The main objective of the expedition was to capture the town of Granada , effectively cutting Spanish America in half and giving Great Britain access to the Pacific Ocean . " " The colours of England , were , in their imagination , already in the walls of Lima . " " Expedition . The expedition , consisting of the transport " Penelope " , two brig s , three sloop s , and a tender , the " Royal George " , sailed from Jamaica on 3 February 1780 , escorted by the 21 - year - old Captain Horatio Nelson in the 28 - gun . Nelson was the highest - ranking officer present , but his authority was limited to naval operations . The overall commander was Captain ( local rank of major ) John Polson , who had at his disposal about 3 , 000 men , including 100 regulars of the 60th Royal American Regiment under himself , 140 of the 79th Liverpool Blues under Captain Richard Bulkeley , 240 Royal Jamaica Volunteers under Major James Macdonald , 250 members of the Jamaica Legion , and 125 of the Royal Batteaux Corps , plus an unspecified number of black volunteers . On 24 March , after many delays awaiting the arrival of several Miskito Indian bowmen under Major James Lawrie , which failed to appear , the expedition anchored off San Juan del Norte . Three days later all Polson ' s troops were assembled aboard boats and advanced upriver divided in two contingents . On 9 April , Nelson , in the first hand - to - hand combat of his career , led an assault which captured a small Spanish battery on Bartola Island . Five miles ( 8 km ) upstream was Fort San Juan , with about 160 armed defenders of whom only 60 were soldiers , which was besieged beginning on 13 April . Because of poor planning and lost supplies , the British soon began to run low on ammunition for the cannons as well as rations for the men . After the tropical rains started on 20 April , men began to sicken and die , probably of malaria and dysentery , and maybe of typhoid fever , Nelson was one of the first to become ill , and he was shipped downriver on 28 April , the day before the Spanish under Juan de Ayssa , devoid of ammunition , food and water , surrendered the fort . The British troops , unable to advance despite the arrival of 450 British reinforcements on 15 May , remained in occupation of the fort for six months , during which time they perished by the hundreds , while Viceroy Matías de Gálvez was able to fortify the mouth of Lake Nicaragua . The Spanish gained in strength , thanks to assistance from San Miguel , Choluteca and other adjoining provinces , while sickness continued to take a heavy toll among the British troops , forcing the order for withdrawal to be given on 30 November . Aftermath . The Spanish reoccupied the remains of the fort , which the British had blown up on departure . Despite having surrendered it , Don Juan de Ayssa was promoted to lieutenant colonel , as was recorded in the Royal Order of 12 June 1781 : he had alerted the other garrisons in the area , bravely defended the fort , suffered hardships as a prisoner of war , and contributed greatly to creating difficulties for the British operations . Taken prisoner to Jamaica , he was freed at the end of the war and rose to become governor of Nicaragua in 1783 .