Journalist James Bamford summarized Operation Northwoods in his April 24, 2001 book Body of Secrets thusly:
Operation Northwoods, which had the written approval of the Chairman and every member of the Joint Chiefs of Staff, called for innocent people to be shot on American streets; for boats carrying refugees fleeing Cuba to be sunk on the high seas; for a wave of violent terrorism to be launched in Washington, D.C., Miami, and elsewhere. People would be framed for bombings they did not commit; planes would be hijacked. Using phony evidence, all of it would be blamed on Castro, thus giving Lemnitzer and his cabal the excuse, as well as the public and international backing, they needed to launch their war.
(From Wikipedia, where you can find lots more info and links on Operation Northwoods. )