Anyone who doesn't say the Civil War needs to retake US History. Not that there weren't other important defining moments, but in sheer impact, the only one that comes even close is the Revolution that started it all. Until the Civil War happened, it was not inevitable we'd even remain one nation, let alone what character this nation would have.
Without the American Revolution we wouldn't have a country in the first place, and without a Constitution there would be no reason to keep the union united