TLDR: Apparently the English wanted to take advantage that Spain joined the American Revolutionary War to try to take over some Spanish towns on Lake Nicaragua. They sent 3000 soldiers up-river, had marginal success in attacking some Spanish forts but yellow fever and diseases wiped out 2500 men.

>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.