Propaganda is a systematic and widespread campaign to support a particular idea or doctrine or to damage an opposing viewpoint. The word propaganda
often carries a negative connotation, implying active deception on the part of the propagandist, but it's really a matter of perspective. One side might proclaim that they spread only the truth, while their opposition spreads deceptive propaganda. But the opposing side says the same thing.
Propaganda campaigns are common during times of war. During World War II, for example, the U.S. government launched a successful propaganda campaign to put homemakers to work while their husbands fought overseas. The centerpiece of the campaign was a character named Rosie the Riveter, a strong, patriotic woman who was doing her part in the war effort.
In The Jungle, Upton Sinclair describes Tommy Hinds, a fervent believer in and an active member of the Socialist Party, who travels the United States trying to convert anyone he can to socialist ideals. And it doesn't stop when he comes home to Chicago:
Hinds's hotel was a very hot-bed of the propaganda; all the employees were party men, and if they were not when they came, they were quite certain to be before they went away.