I totally agree with all of the wonderful comments. This film and 'Manchurian Candidate' are my two favorite thrillers. Each is incredibly thoughtful and well directed. Each requires paying attention . . . and is worth it.
Caveat . . Each has had a mediocre re-make. In films, the original is not always the best, but in these cases, they are . . by far.
Comment
The Day of the Jackal