April 12 (UPI) -- On this date in history: In 1861, the Civil War began when Confederate troops opened fire on Fort Sumter in South Carolina. In 1945, Franklin Delano Roosevelt, the longest-serving president in U.S. history, died of a cerebral hemorrhage at Warm Springs, Ga. , three months into his fourth term. About 3 hours later, Vice President Harry S. Truman was sworn in as chief executive . In 1955, U.S. health officials announced that the polio vaccine developed by Dr. Jonas Salk was "