Here's my three major events that were turning points in American history: The Revolutionary War, World War II, and the atomic bomb. The Revolutionary War was a turning point because if we hadn't fought it, we might still be part of Great Britain and would never have become an independent and free nation. World War II was a turning point because if we hadn't fought it and won, we might all be speaking German or Japanese. Those two nations were cruel, ruthless, and determined to overrun the whole world. We had to fight them to protect our nation, our way of life, our freedoms. The creation of the atomic bomb was a turning point because a whole new power was discovered that when unleashed, could destroy everything in its path. The advent of the nuclear age has ushered in a whole new era and we'll never be the same again.
A good book to reference is Turning Points: Making Decisions in American History by David Burner and Anthony Marcus.