President McKinley didn't want to get in the War at the time, but some things happened which pushed him into joining. Once reason was the fact our battleship, U.S.S. Maine, was sunk. Spain wasn't proven to be involved, but USA had good reason to believe that they were. Yellow Journalism was also a pretty big factor.
Although you only asked for two, I'm just going to give you the other two reasons as well. The United States supported Cuba's desire to have independence so we wanted to go to War for them so they could enjoy like we were. And, lastly, we wanted to protect our business interests with Cuba.
