A must-read book about propaganda and rhetoric is Evonne's Levy's admirable Rhetoric and the Jesuit Baroque.
It poses a provocative question: What is the difference between rhetoric -- classical, Aristotelian rhetoric -- and 20th-century propaganda?
Reflecting on her comments, and fully aware of my immense intellectual limitations (I don't read Greek and am no philosopher) on this subject, I now venture to suggest the following:
Tone: Aristotelian rhetoric is even-tempered. Twentieth-century propaganda is marked by stridency, irrationality and violence to language.
Audience: Aristotelian rhetoric, while not overlooking the importance of human feelings (1), is an appeal to those who think and can control their dog-eat-dog instincts. Twentieth-century propaganda aims to manipulate the crowd by firing-up atavistic emotions.
Purpose: Propaganda, let's face it, exists for one purpose and one purpose only: for the benefit of the propagandist and/or her organization (nothing wrong with that, by the way; but it's how you do it). Aristotelian rhetoric does, however, "reach out" beyond the propagandist for the benefit of the polis (here I am speculating; perhaps I am being overly kind to politicians, even of the best sort). And, course, there is the question of religious, specifically Catholic, propaganda during the Counter-Reformation: to whose "benefit" was it carried out? The Church as an organization? Or maybe, in the eyes of devoted priests, for the benefit of God? Or even more altruistically, for the benefit of the heathen themselves?).
Tools: The spoken word in all its purity and complexity is the essence of Aristotelian rhetoric. The brutal amplication of sound and images for purposes of simplification and therefore manipulation marks 20th-century propaganda.
Walter Isaacson, who's decided to leave his position as Chairman of the Broadcasting Board of Governors, characterized the Declaration of Independence as "in effect, a work of propaganda -- or, to put it more politely, an exercise in public diplomacy."
I am not quite sure whether our (American, but maybe universal?) Declaration -- a late-18th century document, reflecting the past but forecasting the future -- reflects classical rhetoric or 20th century propaganda. Perhaps, the Declaration is indeed propaganda, in the sense that in its putative appeal to the values of mankind, it sought to change the behavior of a refined but parochial Euro-America world (or, more specifically put, the enlighted "Republic of Letters" of that community) for the benefit of the rebellious colonies -- i.e., to get support for American independence for the elite in that narrow but geographical extensive world (reminds me of the Internet).
Give credit to Isaacson: The Declaration, like all propaganda, benign or crude, was a me-first statement: it aimed to win over a "candid" world in favor of American independence by listing "facts" so that a white-skinned, male elite of property-owners in the American colonies could be "independent" -- arguably and more crudely put, didn't want others like themselves in the British Empire controlling/taxing them.
Be that as it may, the Declaration, for all its impolite (but, arguably, justified) polemic against George III and attacks on Native Americans, is far more civilized in its politics than the crudity of much of 20th-century propaganda, and not only that of totalitarian states.
As for propaganda in our new century, it is an illusion to believe that the "social media" mean an end to a phenomenon that has existed since the begining of mankind, propaganda. The purpose of propaganda is, essentially, "Do as I say." And that aspiration (illusion) on the part of (usually) elites, so many of them self-proclaimed, has existed since Adam and Eve, in various forms: some crude, some sophiscated.
Just ask Jared Cohen or Alec Ross.
I can see Twitter as the perfect tool for 21st-century dictators. Just read Orwell and 1984's telescreen.
(1) From Aristotle's Rhetoric, Stanford Encyclopedia of Philosophy: "Aristotle joins Plato in criticizing contemporary manuals of rhetoric. But how does he manage to distinguish his own project from the criticized manuals? The general idea seems to be this: Previous theorists of rhetoric gave most of their attention to methods outside the subject; they taught how to slander, how to arouse emotions in the audience, or how to distract the attention of the hearers from the subject. This style of rhetoric promotes a situation in which juries and assemblies no longer form rational judgments about the given issues, but surrender to the litigants. Aristotelian rhetoric is different in this respect: it is centered on the rhetorical kind of proof, the enthymeme (see below §6), which is called the most important means of persuasion. Since people are most strongly convinced when they suppose that something has been proven (Rhet. I.1, 1355a5f.), there is no need for the orator to confuse or distract the audience by the use of emotional appeals, etc."