Espionage occurs in societies at peace and at war. Nations at peace use spies to gather information on a country's military preparations and plans for war. During war espionage is used to gather information about opposing armies and to mislead opponents through counterintelligence. Those engaged in spying risk imprisonment and death if convicted of treason. Although vital to ensure American security in war and peace, spying—part of intelligence gathering—raises important issues such as the extent civil liberties may be reduced to allow the government to catch spies. The American Civil War marked the beginning of extensive civil and military espionage. The intelligence operations during the Civil War were pre-modern, amateurish, and even eccentric by twenty-first century standards, but elements of this odd secret war foreshadowed the later modernization of America's novice intelligence services.

Although spies could be useful to resourceful commanders, in...