On May 1, 1919--May Day--postal officials discovered 20 bombs in the mail of prominent capitalists, including John D. Rockefeller and J.P. Morgan, Jr., as well as government officials like Supreme Court Justice Oliver Wendell Holmes. A month later, bombs exploded in eight American cities.On September 16, 1920, a bomb left in a parked horse-drawn wagon exploded near Wall Street in Manhattan's financial district, killing 30 people and injuring hundreds. The suspicion was that the bomb was the work of alien radicals. Authorities...