However, the single most influential and newspaper-exploited event in the process of bringing the United States into a state of war was media coverage of the mysterious and unexplained explosion of the U.S.S. Maine. Immediately after, many newspapers (particularly those such as Hearst's Journal that commonly practiced certain aspects of yellow journalism) carried headlines such as "Remember the Maine!" and articles immediately accusing the Spanish for the destruction. Some even went so far as to make up detailed stories, stressing that it must have been a mine or torpedo (delivered by, of course, the Spanish) that caused the deaths of two-hundred and fifty-two American soldiers. Within days, headlines became so blunt as to say "War? Sure!" With pressure on the government from the people, the press, and eventually even on certain parts of the government itself, a state of war came into effect on April 25, 1898 (made retroactive to April 22, 1898). Armies were mobilized, emergency funds were allocated, and ports were blockaded, marking the beginning of the Spanish-American War.
While the truth about what happened the U.S.S. Maine is still unknown (theories range from completely accidental internal explosions to Spanish torpedo fire to a mine set by agents of the "yellow press"), one can be relatively sure that this war may very well have not happened without the encouragement of the propaganda put out by newspapers all over America. To make this point more blatant, imagine what would have happened if the "yellow kids" (Hearst and Pulitzer) had strongly disapproved of the war, for whatever reason. Spanish brutality would be toned down, seen as a non-issue, and the destruction of the Maine would have been promoted as an accident. Basically, due to the public opinion that they had such thorough control over, propaganda was the decisive factor in starting this war. So, overall, this becomes one of the most significant and representative events in the history of modern propaganda.
http://library.thinkquest.org/C0111500/spanamer/app.htm