Harman Blennerhassett was an Anglo-Irish lawyer, a member of the Society of United Irishmen who emigrated in advance of their rebellion in 1798 to become a socially and politically distinguished plantation owner in then-western Virginia. Implicated in the Burr conspiracy, an alleged military plot with Britain to separate the Louisiana Territory from the American Union, he was twice arrested and financially ruined. His last years were spent in England.

The Burr conspiracy was a plot alleged to have been planned by Aaron Burr in the years during and after his term as Vice President of the United States under US President Thomas Jefferson. According to the accusations against Burr, he attempted to use his international connections and support from a cabal of US planters, politicians, and army officers to establish an independent country in the Southwestern United States, parts of Mexico, and Florida. Burr's version was that he intended to farm 40,000 acres (160 km2) in the Texas Territory which had been leased to him by the Spanish Crown.