From an essay I'm working on for my history class:
In 1898, the Spanish and Americans were already jousting over control of Cuba. The issue was that the more affluent Spanish were mistreating the poor of Cuba, and colonial rule was causing death and famine among the Cuban natives. Some of this was true, but much of it was greatly exaggerated or even completely invented by newspaper editors in an effort to draw greater readership. At this time, there were two main newspaper publishers: the up-and-coming William Randolph Hearst, and old hand Joseph Pulitzer. They were locked in a struggle for control of the press in major cities – most notably New York City. Without regard for the consequences of their actions, they commissioned often outrageous articles about the mistreatment of Cubans by the Spanish spun more for entertainment than for the value of their information in hopes of selling more papers. As eldritch said, they did include comics such as "The Yellow Kid," and they also bought work from many notable artists of the time depicting the atrocities being visited upon the Cuban poor. The tactic worked, and papers flew off of the stands. But by the time the USS Maine sunk on February 5 of 1898 – an event some say may not have been the fault of the Spanish at all – anti-Spanish sentiment had boiled so high then-President McKinley could not refuse to start a war without ruining his political career. Thus yellow journalism was a chief cause of the Spanish-American war. Thanks to the outlandish publicizing of the media moguls, dubbed "yellow journalism," the war was made inevitable.
Yellow journalism may have been a term applied to the "reefer madness" campaign of the 1930s, but it was applied as a derogatory term with historical connotations ... extending as far back as 1896.