Believe it or not, both revisionist historians like Howard Zinn and traditional historians like Larry Schweikart point out that the influence of Yellow Journalism on the causes of the Spanish Civil War have been overhyped. In the bumper sticker approach to US History, this makes for an easy cause, but the reality is that business interests had a much bigger role in starting the Spanish American War than supposed Yellow Journalism.
The main impact that “yellow journalism” had on the Spanish American War was to push the United States towards getting involved in that war. William Randolph Hearst is famous for having used his newspapers to push for American involvement in the war.
Hearst and his papers used yellow journalism to push the US into the war by constantly trying to make the Spanish look as bad as possible. They started out by hyping what they saw as Spanish atrocities. The Spanish were fighting against a Cuban rebel insurgency and the yellow journalists played up things like the Spanish concentration camps. By doing so, they inflamed American opinion against Spain. The yellow journalists were especially effective in pushing the US into the war after the sinking of the USS Maine in the harbor of Havana, Cuba. The newspapers asserted that it was a foregone conclusion that Spain had attacked the ship. There were even illustrations purporting to prove how Spain had done so. There was no basis for these claims, but they were tremendously important in pushing the US into war.
Thus, yellow journalism was mainly important with regard to this war because it inflamed American public opinion, causing people to pressure the government to go to war against Spain.