You are an intelligent, intuitive and inquisitive person, and I'm sorry people are being so nasty. Unfortunately, high school AMerican History courses usually start with Columbus "discovering" America and don't get through the entire book, in other words, the parts where they discuss modern history, WW I, WW II, Vietnam and all that. It's not your fault. Just bad planning and curriculums at the schools. People have given you some good stating points tonight and you should follow up on them. If you go to college make a point of taking a modern US History course. They usually from 1900 on. And you will find that many students have had the same lack of experience in this area as you had.
I'm guessing you heard something about World War II, maybe on TV? The fact that you immediately asked the right questions, as in, well, there must have been a World WAr I, tells me that you are an intelligent person as I said in the first paragraph above. So don't listen to the snotty comments made to you about this question.
In the meantime, here's a quick hint if you want to sound intelligent in a conversation about the world wars. Just say, "It's obvious to me that World WAr II was directly caused by the Allies severely punishing Germany at the end of World War I.