Why have there been no terrorist attacks in the United States since 9/11? It is ridiculously easy for a single person with a bomb-filled backpack, or a single explosives-laden automobile, to launch an attack. So why hasn't it happened? The answer is surely not the Department of Homeland Security, which cannot stop terrorists from entering the country, legally or otherwise. It is surely not the Iraq war, which has stoked the hatred of Muslim extremists around the world and wasted many thousands of lives. Terrorist attacks have been regular events for many years -- usually killing handfuls of...