Most Important Spies in US History
A few years back during a university lecture, I asked the students to name the US traitor whose espionage had done the most damage to our country; one the most important spies in US History. I was expecting them to name Edward Snowden. After a bit of thought one of them threw out the name Benedict Arnold. That surprised me.
Arnold was a traitor, but he wasn’t much of a spy. He did pass some information to the British on the defenses at West Point, but when he tried to help them take that installation, the operation ended with his handler, British Major John André, captured and hanged, and Arnold running for his life.He ultimately had to flee the colonies as Washington wanted his former friend and subordinate arrested and executed for his perfidy. Famous traitor and damaging spy aren’t synonymous terms.
If Arnold wasn’t one of the most important spies in US history, who was? Here’s my Top 5 list of candidates for most important spies in US History.
5. Robert Hanssen. The scope of Robert Hanssen’s espionage is mind-boggling, as is the cold-blooded approach he took to it. An FBI counterintelligence officer, Hanssen gave up to Moscow the names of all Russian double agents working for the US, resulting in the executions of at least four including General Dmitri Polyakov and several KGB officers. He warned the Kremlin about the investigation into Felix Bloch, compromising another major espionage investigation and ultimately allowing Bloch to escape prosecution. The financial loss of sources and methods has been estimated far into the billions of dollars, but the most damage may have come when Hanssen leaked information about our “continuity of government” system. Upon the outbreak of war, the Russians might well have been able to decapitate the entire United States government. Thankfully, that never happened, but for making it theoretically possible for the Soviet Union to cripple the entire US military leadership structure during wartime, Robert Hanssen earns my #5 on the list of most important spies in US History.
4. John Anthony Walker. Walker was arrested for burglary as a youth in 1955 and given the option of jail or the US Navy. He chose the latter, but what happened later should cause judges to reconsider giving any defendant that option. By the late ’60s, Walker was deep in debt and decided that espionage was the solution. He marched into the Soviet Embassy in Washington, sold them a Navy radio cipher card to establish his bona fides, and over the next eight years gave Russians classified information on all of the US Navy’s communications systems. Vitaly Yurchenko, a former senior KGB official, later said that Walker had so thoroughly compromised the US Navy’s communications security that “if there had been a war, we would have won.”
In 1976, worried that the heat was on, Walker resigned from the Navy, but he was addicted to the Russians’ salary—several thousand dollars per month. So he convinced his brother Arthur, a military contractor and retired naval officer, to pass him classified information; he also pressured his son Michael to join the Navy explicitly to commit espionage. Unfortunately for John, his divorce that same year was a bitter one, his wife knew everything, and she eventually reported him to the FBI in 1984.
It’s estimated that the Russians paid Walker over a million dollars during his time as a spy. It was a bargain. Walker’s espionage most certainly would have led to the deaths of thousands of US sailors and the destruction of numerous US vessels in a war with the Soviets, which, again, thankfully didn’t happen.
I think it’s one of the great ironies of espionage history that John Walker, who had made a living using his voice as a naval communications officer, died of throat cancer while serving a life sentence in North Carolina’s Butner Federal Correctional Complex in August 2014, one year before he would have been eligible for parole.
3 and 2 (tie). Julius and Ethel Rosenberg, and David Greenglass. There is no doubt that after the US destroyed Hiroshima and Nagasaki, the Russians or some other nation would eventually have learned or stolen the secrets to building atomic weapons. Enemies of the United States would have spared no expense to develop their own nuclear capability. But the Rosenbergs and Greenglass helped make sure of it, and the Russians had The Bomb by August 1949.
Julius, a member of the Communist Party in the 1930s, was recruited in 1942 by the NKGB—the KGB’s predecessor—while he was working for the US Army Signal Corps. When his Russian handler, Semyon Semyonov, learned that Ethel’s brother, David, worked at Los Alamos on the Manhattan Project, he directed the Rosenbergs to recruit David. Julius succeeded and Greenglass began passing atomic secrets to the Rosenbergs and Semyonov through courier Harry Gold.
Gold was also the contact for another important spy—Karl Fuchs, a German theoretical physicist and naturalized British citizen assigned to the Manhattan Project. Fuchs stayed on at Los Alamos after the war, working on other nuclear weapons programs and passing information on all of it to the Soviets through Gold. By late 1949, Britain’s GCHQ figured out through its VENONA signals intercept program that Fuchs was a spy. Arrested in London, Fuchs identified Gold as his contact; Gold was arrested and identified Greenglass; Greenglass was arrested and identified Julius and Ethel.
Greenglass earned a 15-year prison sentence and served 9—he died in 2014 in a retirement home where he’d lived under an assumed name. The Rosenbergs were executed in 1953 and remain, to this day, the only US civilians ever to receive the death penalty for espionage—a still-controversial sentence. 
But the fact is that Greenglass and the Rosenbergs, with an assist from Fuchs, were the world’s first nuclear proliferators.
1. Henry Thomas Harrison. Harrison was an actor until the Civil War started when he and the Confederacy found a new use for his theatrical talents, and his new career in espionage turned the course of the entire war in the Eastern Theater. In June 1863, General Robert E. Lee’s chief cavalry officer, Jeb Stuart, was humiliated when Union cavalry surprised him at Brandy Station where Stuart had been too preoccupied with staging a mock battle for Lee and the locals to conduct a proper reconnaissance of the area. Determined to save his reputation, Stuart went off raiding around Washington, leaving Lee blind to Union movements as the Army of Northern Virginia moved north into Pennsylvania. As late as two days before the Battle of Gettysburg, Lee thought the Union Army was still in Virginia, at least sixty miles distant. Feeling safe, he spread his army over the Pennsylvania countryside, marching separate roads and raiding towns for supplies.
Enter Harrison. Hired by Lee’s chief lieutenant, General James “Pete” Longstreet, Harrison had been gathering intelligence in and around Washington, DC, and Maryland during the weeks before. On June 28, he showed up with a report that the Union Army had crossed the Potomac River three days earlier and was as close as Frederick, Maryland—little more than a hard day’s march.
Without Harrison’s report, the Union Army might have hit Lee’s army while it was spread out across Pennsylvania, destroying it corps by corps, division by division, possibly ending the war. But, now forewarned, Lee pulled the Army of Northern Virginia together just in time to fight the Battle of Gettysburg—the largest and bloodiest battle of the war—two days later. Lee lost the battle and 22,625 casualties, but he returned to Virginia with two-thirds of his army intact. Courtesy of Henry Thomas Harrison’s last-minute save of Lee’s army, the war would continue for another twenty-one months, during which hundreds of thousands of men would die and devastation would spread across the South that still affects the country today.
It might seem strange to name a Civil War figure as the most damaging spy in US history, but where the espionage activities of Hanssen, Walker, the Rosenbergs, and Fuchs might have eventually led to incalculable American deaths and destruction, Harrison’s actually did. If there’s ever a war between the US and Russia, I’ll have to revisit this list.
And whither Edward Snowden? The damage assessment on his treason is still ongoing, so we don’t know where he falls in the pantheon of American traitors. That said, I’m sure he’ll land somewhere near the top when the jury’s verdict finally comes back.
*Mark Henshaw is currently a CIA analyst and the author of several thrillers including his latest, The Last Man in Tehran (Touchstone, December 26, 2017).