Per the story, this was not to stage a coup but in the event of a Left-wing Coup in Brazil, per the force, we would have intervened. I guess, not much became of it.
U.S. Assembled a Force in 1964 For Possible Use in Brazil Coup
By DAVID BINDER DEC. 30, 1976
WASHINGTON, Dec. 29—The Administation of President Lyndon B. Johnson prepared briefly to intervene in Brazil with naval and airborne units in 1964 to prevent leftist forces from seizing power in the wake of an expected millary coup, according to recently released documents and to the former United States Ambassador, Lincoln Gordon.
A naval task force was assembled March 31 in the Caribbean and started south toward Brazil it was disclosed in documents at the Lyndon B. Johnson Library in Austin, Tex. The documents were removed from secret status early this) year.
Mr. Gordon who is on a skiing vacation in New Hampshire, confirmed by telephone that he had proposed the creation of the intervention force He said:
Read more at: https://www.nytimes.com/1976/12/30/archives/us-assembled-a-force-in-1964-for-possible-use-in-brazil-coup.html
