Hypothetical: Lee Harvey Oswald's bullets miss JFK and Kennedy survives Dallas. Would Kennedy win a second term and, if so, what would Kennedy's second term be like? Would he had pulled out of Vietnam? Would the world be different today?
Side Note: Stephen King actually has a recent book out on this topic, but because it has alot of science fiction elements in the book, it doesn't really deal with these issues realistically.
Another Side Note: On that fateful day in Dallas, Kennedy wore an extra strong brace because he had injured his groin the day before, cavorting with some girlfriends. This brace held his body in a stiff, rigid posture, making him an excellent target for Oswald. Had there been no extracurricular activities the day before, Oswald might have missed or non-fatally injured him with only the first bullet.