The Spanish-American War started off with no conflict between US and Spain. Cuba and Spain were were arguing because Cuba thought that it should be it's own country and Spain didn't. Spain was basically using harsh and cruel punishment to get Cuba to shut up. The US wasn't happy, but they didn't really care at first. The US believed that the people should be able to decide who rules them.
America gradually came into the picture when Pulitzer and Hearst published exaggerated stories about the conflict in the Yellow Press. More and more Americans demanded war. The Yellow Press wanted the war so that they could sell more newspapers.
The straw that broke the camel's back was when the Yellow Press published the de Lome letter; which was a letter sent from a Spain man in America his friend in Cuba to reassure him that there would be no war. He said that McKinley was a coward. American's were outraged at this. But that was not it. When the U.S.S. Maine was blown up; killing more than 260 soldiers, Americans, if they hadn't before, they were now demanding war more than ever. They blamed the explosion on Spain and told McKinley to go to war.
McKinley had decided to go to war even though Spain had given in to America's demands. He didn't want to look like a coward to the American people. He was also afraid that resisting war would cost Republicans control of Congress in the fall elections.
The battle for the Philippines was very important to the war. This is where Spain's fleet was. The US took it out. This was very important because now, Spain only had its land crew. This was one of the factors that ended the war.
The US military was far too small. However, there were many voluntary groups that were able to go to war for their country. One of which was led be Teddy Roosevelt was the Rough Riders. They were a voluntary Calvary unit that had no skills. They were consisted of cow hands, business personalities, upper-class, pro-athletes, and even ex-cons. They would have been butchered, however, the black regiment helped them and were responsible for the win of the battle of San Juan Hill. The black soldiers also played a key role in the Philippines. San Juan Hill was important because this is where we pushed back the Spanish to Santiago where they tried to slip out, but were intercepted and destroyed.
The war said to the world that America was not going to stand for unfair punishment from one country to another. It said that if a country wanted freedom, they should try and take it. But that is just my opinion.