I have outlined the world events that provide context for the intellectual history being made as well as examined propaganda through various theories. The sources that I've used to examine propaganda are in the top layer.
Le Bon states that as the psychological crowd gathers, they experience a collective intellectual regression. While this part of Le Bon's theory does not have much merit his ideas on crowd hypnosis may have some weight, "All feelings and thoughts are bent in the direction determined by the hypnotiser." . Le Bon's ideas on the hypnosis of crowds essentially is the goal of propaganda to control groups of people without them knowing it.
1927- Harold Lasswell states that propaganda manipulates the masses to ensure the one entity maintains political authority and agendas. "If we state the strategy of propaganda in cultural terms we may say that it involves the presentation of an object in a culture in such a manner that certain cultural attitudes will be organized toward it" . We see propaganda in action with nationalistic slogans or imagery that plays on the public's fears. For example, images of Japan or Nazi Germany subjugating the U.S.
Bernays is all for the use of propaganda to manipulate groups of people in a democratic society. "...If we understand the mechanism and motives of the group mind, is it not possible to control and regiment the masses according to our will without their knowing it? ... The recent practice of propaganda has proved that it is possible..." . With the mention of the group mind it is evident that Bernays is pulling from Freud's theory on the Primal Horde.
Those who have differing opinions will be pushed into silence. Hence the name "spiral of silence". How these people know that their opinion is not welcome is through the use of their "Quasi-Statistical Organ".  This theory can perhaps explain why Nazi Germany propaganda was so potent with its effects. Those who had dissenting opinions on German public policy and German society at the time were likely forced into a spiral of silence because they perceived the hatred of Jews and German superiority as the general public opinion.
Signifier = A man in swastiska underwear
Signified = Hitler being caught by surprise in his underwear
The combination of the signifier and the signified compose the sign. The sign creates a connotation and within the context of World War 2 the connotation is meant to encourage American men to enlist and for those who are not eligible to join the military to keep doing what they can on the home front to support the troops. 
Provides a comprehensive look at propaganda overall and its use in Germany, Britain and the U.S. This webpage from PBS allows the reader to gain a sense of how German propaganda produced by Joseph Goebbels was used to increase morale and keep up the idea that Germany was supreme. The article then goes on to describe how propaganda from America and Britain, discouraged the German people so propaganda can be used against an enemy as well not just to boost one's own country's morale. 
Framing is how a news story or topic is talked about while agenda setting is how much attention a subject gets in the media  An aspect of propaganda which may have made it more powerful was how framing and agenda setting were done. In the U.S. the media framed the Axis powers in a negative way which made the receiving audience view countries like Japan and Germany in a negative light. The framing of the Axis powers was spread through the use of agenda setting, almost all forms of media carried the same or a very similar message, cartoons, radio shows, posters, and newspapers. With the war having this amount of presence in multiple media formats it makes sense why war propaganda made such an impact.
Pooley essentially gives the reader a comprehensive review of the old and new history mass communications research. In his evaluation of the new history he looks at how World War 2 heralded the fast tracking of communications research due to the fact that the U.S. wanted to counter Nazi propaganda. But, the desire for communication studies was also to help produce American propaganda to raise support for the war effort. 
Stout focuses in on Nazi propaganda and looks at Joseph Goebbels journals as well as other documents from the time that the Nazi party was in power Stout is able to draw his conclusion. Stout concludes that propaganda was extremely effective through the use of 3 elements:
While this article focuses on Nazi propaganda I think it speaks to how potent propaganda can be when it is well placed and well done.
Freud focuses on the effect of being in a group on the individual and comes to the conclusion that in large groups the individual suffers undergoes a regression. This regression takes the individual back to the Primal Horde who are led by the Primal Father. 
Messages sent by media producers are received and immediately accepted by the receiver. (This is a largely obsolete theory today).
Essentially, Adorno is arguing that popular culture is being produced by the culture industry in order to make a profit. To clarify, the "culture industry" is when institutions create and market something (with Adorno it's music) to the masses to make money off of it. Adorno is saying that the culture industry is detracting from individuality. 
Stated the mass communications (like propaganda) actually had weak effect on changing or manipulating the masses, "...the finding that radio and the printed page seemed to have only negligible effects..." . Lazarsfeld is essentially saying that interpersonal communication is what truly influences people's decisions. Going off of what was briefly covered in the Payne Fund article, Lazarsfeld asserts that mass media only has a "weak effect" on the masses he then introduces the "Two Step Flow of Communication":
Braddock built on Lasswell's Model:
Who --> Says What --> In Which Channel --> To Whom --> For What Purpose --> Under What Circumstance --> With What Effect
Adorno defines the "culture industry" as the products of culture from institutions seeking to make a profit and then goes on the redefine "mass culture" as something driven by the people who create something for themselves that is not constructed by an institution. Adorno believes that cultural forms create a means of income for their creators, so profit has become more important than the artistic expression. So therefore, culture has turned into an industry and the cultural objects are looked at as products. 
Davison discusses the "third-person effect" in regards to propaganda and how those who are exposed to persuasive mass communication will believe that the mass media will have more of an impact on others than themselves. 
Daniel McDonald looks at how The Payne Fund Studies led into interest on how effective persuasive mass communication was in manipulating the masses. From the Payne Fund Studies, researchers learned that media can affect people and from here researchers began to look into radio and the War of the Worlds phenomenon, now there was more evidence that propaganda and media could have a heavy impact. This led to the effects research conducted by Lazarsfeld, Cantril, Allport, and Hovland and found that mass media only has limited effects.