Which number should replace the question mark?

Bob had finally made it to the last round of the $5,000,000 Question. The night before the big question, he told the M.C. that he desired a question on American History.

The big night had arrived. Bob made his way on stage in front of the studio and TV audience. He had become the talk of the week. He was the best guest this show had ever seen. The M.C. stepped up to the mic.

"Bob, you have chosen American History as your final question. You know that if you correctly answer this question, you will walk away $5,000,000 dollars richer. Are you ready?"

Bob nodded with a cocky confidence-the crowd went nuts. He hadn't missed a question all week.

"Bob, your question on American History is a two-part question. As you know, you may answer either part first. As a rule, the second half of the question is always easier. Which part would you like to take a stab at first?"

Bob was now becoming more noticeably nervous. He couldn't believe it, but he was drawing a blank. American History was his easiest subject, but he played it safe. "I'll try the easier part first."

The M.C. nodded approvingly. "Here we go Bob. I will ask you the second half first, then the first half."

The audience silenced with gross anticipation . . .

"Bob, here is your question: And in what year did it happen??"