There are many ways to look at graphic design. One can say it is an art of creating appealing imagery with text and pictures, another one can say it's a technical skill of arranging imagery and text while preserving legibility and readability. Either one would be correct, but I also think there is a possibility of using graphic design to persuade audience using effective design principles that can create certain public opinion.
A primary purpose of graphic design is to deliver some information and communicate certain messages. These principles were taken in consideration and developed further by the governments that used graphic design to lead their citizens in a needed direction, to promote goals and desired attitudes. This idea is known today as propaganda. Through use of propaganda posters governments have been shaping public opinion throughout many years, but only in 20th century its influence was critically examined and developed into communication theory. There were many theorist that approached this topic, but I mostly admire a work by Walter Lippmann called Public Opinion(1922). His book contains a lot of practical examples that explain origins of communication and influence of public opinion through media and propaganda in particular.
Although his work is an incredible source to refer to, this blog doesn't focus specifically on his work, but takes a look at communication theory and its application to graphic design in particular. I think as graphic designers we sometimes don't realize what kind of power we have in our hands when we design a poster, a flyer, a logo, a websites or anything else that intends to deliver messages and promote ideas. I specifically take a look at propaganda posters of early 20th century and contemporary American advertising and examine what design principles they use to persuade their intended audience.