Propaganda Piece

Propaganda is generally defined as the communication of ideas, information, and rumors for the purpose of influencing opinions in support for or against a particular cause through emotional appeals; the Propaganda Piece (which is also referred to as just 'propaganda') is the means by which this is carried out.

In short, it is effectively a deliberate Author Tract, which naturally means there is a lot of overlap between the trope. Author Tracts are distinct in that they tend to be more personal to the creator in question, and can often be an unintended result of their choice in themes, though it can also be very much intentional. Conversely, the Propaganda Piece is usually far less ambiguous (if at all) about its purpose as such; it is often the result of an effort to garner support for an organization or institution, and is frequently created in some part or else commissioned by that organization or institution themselves.

Despite the unsavory and manipulative connotations associated with the term in modern times, "propaganda" is considered a historically neutral term. And as always, Tropes Are Not Bad - for example, religions are naturally inclined towards spreading news of their faith as part of their function. There is also some overlap with Public Service Announcements, which are designed to increase public attention towards matters of health and safety - as is the case with many a form of Anvilicious media, Some Anvils Need to Be Dropped.

Comic Books

 * Captain America originated as a comic intended to rally support against the Nazis from a non-intervening America; the famous issue #1 cover shows Cap in his iconic red-white-and-blue costume punching Adolf Hitler, and post-WWII would see him fighting Communists accordingly.
 * Jack Chick's infamous Chick Tracts, free comics intended to disseminate the author's radical Christian views.

Literature

 * Uncle Tom's Cabin was written by Harriet Beecher Stowe in order to further abolitionism through showing the evils of slavery.

Western Animation

 * The entire purpose of Wartime Cartoons.