Since the Revolutionary War, America's military and political leaders have recognized that U.S. national security depends upon the collection of intelligence. Absent information about foreign threats, the thinking went, the country and its citizens stood in great peril. To address this, the Courts and Congress have historically given the President broad leeway to obtain foreign intelligence. But in order to find information about an individual in the United States, the executive branch had to demonstrate that the person was an agent of a foreign power. Today, that barrier no longer exists....