His name was William Walker. There are several secondary sources about his activities, some of which are in the public domain. The link below is his own work about Nicaragua. These events had little to no impact on the U.S. after the War.
https://books.google.com/books?id=XrcWAAAAYAAJ&printsec=frontcover&dq=william+walker+filibuster&hl=en&sa=X&ei=SrfTVM20DsWgNp2Jg-AD&ved=0CCsQ6AEwAg#v=onepage&q=william%20walker%20filibuster&f=false
