In amplifying the previous posts, I would suggest that the passage of the United States Espionage Act in 1917 represented a severe measure to control public opinion after the United States had entered the First World War. The idea of being able control dissent or any opinion that "is to be used to the injury of the United States" represented one of the most lucid examples of how war is "the health of the state." In this light, a nation that entered the war on the premise of "making the world safe for democracy" had to answer on its own domestic front for its suppression of it. The idea of being able to pass such legislation by a wide majority, enforce it to a great extent, and bolster it with the preponderance of pro- war propaganda helped to drive home the fact that war centers on the consolidation of Status Quo power.
One medium that was used for positive propaganda during World War I was the poster. There were a myriad of posters published during this time; most contained a slogan on them. One example shows a bulldog with a helmet (the US Marines) chasing a German dashund and the words "Teufel Hunden" at the top--the German nickname for the Marines. Another poster is also a recruiting poster that reads:
If you are an electrician, mechanic, or telegraph operator, you belong in the U.S. Army Signal Corps. If you are not, we will train you; get in now.
A motivational poster that was hung throughout the country was one reading, "American Red Cross--Our boys need sox--Knit your bit. One suggests that women buy war bonds; it has Joan of Arc upon it, brandishing a sword.
In the United States during the First World War, the government took some pretty severe measures to control the domestic scene.
First, they took proactive action to shape opinion. They formed the CPI to put out propaganda that would be helpful to the war effort.
Second, they used negative actions -- laws banning various forms of speech and action that could be seen as anti-war. The most famous of these was the law that was applied in the case of Eugene Debs. Debs was jailed for speaking out against the draft.
Most wartime governments take similar actions to control opinion. They limit people's gathering ability, they limit the hours one can be outdoors, and they limit access to information from and to people. By limiting people's ability to assemble, it helps stop the spread of public dissemination of information that can help people form rational and logical opinions about certain things. By limiting people's awareness of other information, it also helps the spread of public criticism. By confiscating radios and televisions, they also limit the ability of people to receive fair and unbiased reports of what is going on.