 This secured the southern route for supplies and closed off the possibility of any British offensive into the western frontier of United States via the Mississippi River.When Spain entered the war, Britain also went on the offensive in the Caribbean, planning an expedition against Spanish Nicaragua.A British attempt to gain a foothold at San Fernando de Omoa was rebuffed in October 1779, and an expedition in 1780 against Fort San Juan in Nicaragua was at first successful, but yellow fever and other tropical diseases wiped out most of the force, which then withdrew and returned to Jamaica.