The world wars. Mainly World War II when the United States earned the name the arsenal of democracy. At that time, Americans were beginning to believe that the world NEEDED the USA and that democracy had to spread throughout the globe. This led to the Korean and Vietnam War because American citizens felt obligated to be involved, at least in the beginning.
John Brown's raid on Harpers Ferry (also known as John Brown's raid or The raid on Harpers Ferry) was an effort by white abolitionist John Brown to start an armed slave revolt in 1859.