In 1823, President James Monroe called for European powers to stay out of the Americas. What came to be known as the Monroe Doctrine would influence U.S. foreign policy for generations, and lead to military interventions across the Western Hemisphere. This past week President Donald Trump repeatedly invoked the Doctrine when discussing last week's incursion into Venezuela. Mo Rocca talks with historian Jay Sexton, author of "The Monroe Doctrine: Empire and Nation in Nineteenth-Century America," about how Monroe's words – and how presidents have interpreted them – have affected world history.