In 1876 George Armstrong Custer - the brave, reckless and vain Civil War hero - met an ignoble end at the Battle of the Little Bighorn. And every year, re-enactors in Montana recreate the clash between the 7th Cavalry Regiment and Lakota Sioux and Cheyenne warriors. Mo Rocca visits the Little Bighorn National Park, and talks with historian Nathaniel Philbrick about the soldier most remembered for his darkest hour.