Since the days of the Revolutionary War, America has fought to protect its secrets from internal spies. While most apprehended spies are caught and arrested within just a few years, many have managed to leak invaluable, highly classified information to foreign governments over lengthy espionage careers lasting many years and sometimes multiple decades. (In America and elsewhere, these are the most famous spies in history.)

To determine the worst cases of espionage in U.S. history, 24/7 Tempo reviewed data on espionage by Americans from the Defense Personnel and Security Research Center of the Department of Defense. Native- and foreign-born American spies were ranked based on how long they spied – the total time elapsed between the start date of their espionage activities (estimated in most cases) and the date of their arrest. (Currently, there are at least 14 Americans imprisoned for espionage.)

The motivation for espionage varies widely amongst the worst offenders. While money is a fine motive for any crime, most spies receive less than $10,000 for their activities, according to data compiled by the Department of Defense. Instead, many spies have turned coat in the name of ideology. James Clark, for example, is said to have developed communist sympathies while a student at the University of Wisconsin, inspiring him to provide East Germany with classified documents for a period of at least 10 years.

In other cases, acts of espionage may originate from sheer boredom, or intrigue for the sake of intrigue. Jeffrey Carney, who provided the Stasi police with over 100 top-secret documents while stationed as an Air Force specialist in West Germany, is characterized as a loner who defected after becoming disillusioned with military life. 

Click here to read about the worst cases of espionage in U.S. history

Robert Hanssen, who perpetrated one of the most damaging acts of espionage in U.S. history selling secrets to the Russians from 1979 to 2001, is said to have been drawn by the theatricality of tradecraft, donning black turtlenecks reminiscent of the “golden age” of Cold War spies to conduct dead drops with his handler in his Washington D.C. suburb.

Whatever the motivation, espionage against the United States has had significant geopolitical impact, costing countless lives both directly and indirectly. One potential consequence of Larry Wu-tai Chin’s three-decade tenure as spy against the United States, for example, was to delay the end of the Korean War by a year, according to some experts. Information provided by Robert Hannsen to the Soviet Union and later Russia directly led to the execution of several U.S. assets, among other consequences.