The correct answer to this open question is the following.
The role that propaganda played in World War I was very important because, through propaganda, the warring nations communicated the proper messages to their citizens in order to let them know what was happening in the war front, according to each government version.
For instance, in the case of the United States, the federal government created a specific office where they used propaganda to convey its ideas and version of reality to the American citizens. It was called the Committee on Public Information (CPI), commonly known as the Creel Committee. It was created in 1914 and disappeared in 1917.
The absence of propaganda could have changed the circumstances or the outcome of the war only in the way information was handled. As mentioned above, the federal government tried to control or census the kind of information that was considered to be conveyed to its citizens. In one way, the information was manipulated to inform just what was considered appropriate for the American people.
