2. To protect U.S. business interests in Cuba
3. Yellow Journalism
4. Sinking of the U.S.S. Maine
The main immediate effect of the Spanish-American War (for the United States, at least) was to make the United States an imperial power.
Before this war, the US had had very little in the way of overseas possessions. That changed with this war. The US took effective control of Cuba. It also took complete control of Puerto Rico, Guam, and the Philippines. By taking the Philippines, in particular, the US became an imperial power.