By Jeremy Isaacs and Taylor Downing
COLD WAR series producers

Slipping in and out of the shadows, busy with cipher and with microdot, stealing and concealing, betraying and being betrayed, intelligence agents of the great power blocs risked their lives to obtain and pass on information. Spies came to personify the very image of the Cold War. But how much did they influence its course?

Early Cold War spies -- Kim Philby, Guy Burgess, and Donald Maclean in British intelligence and the Foreign Office -- acted from political conviction. They believed what they were doing was right. Late in the Cold War, Aldrich Ames, the KGB's mole in the CIA, acted only for money. In between, both agencies expanded vastly: the CIA and KGB came to employ thousands. All the while, rival technologies of signals interception, known as Sigint, and of satellite photography -- mechanical, impersonal, increasingly efficient --threatened to render the human spy redundant.

Spies operating before the Cold War began gave the Soviet Union an early advantage. Klaus Fuchs and Ted Hall were scientists employed at Los Alamos on the project to make the atomic bomb. They passed to their Soviet controllers detailed drawings of the implosion method of exploding the bomb. Partly as a result, the Soviet atomic bomb was ready in 1949, two years earlier than expected.

Burgess and Maclean delivered a flow of information on Western policy to Moscow in the 1940s, including the West's intentions on the Marshall Plan. Maclean and Philby had high clearances in British intelligence. Philby revealed details of Western Sigint to his KGB controllers and betrayed agents infiltrating the Balkans, who were apprehended and shot. The effect of this spectacular treachery, the knowledge that the KGB had an officer in the heart of Western intelligence, was debilitating and dismaying. Americans would not trust British intelligence for 20 years. Burgess and Maclean defected to Russia in 1951; Philby, under suspicion for another decade, finally defected in 1963.

Agents on both sides told their employers what they wanted to hear. Consistent U.S. overestimates of Soviet missile strength fueled the high defense expenditures and spiraling programs of the 1950s and 1960s. But the CIA failed to predict North Korea's attack on South Korea in June 1950, or China's entry into the Korean War in November of that year. The CIA had the information needed, but did not want to believe it. The U.S.S.R., it reasoned, was known not to want to get involved. China was a client of the U.S.S.R.; therefore China would not involve itself either. And the CIA's aerial intelligence could not then reach farther than an enemy's border. The CIA's Korean debacle led to the setting up of the National Security Agency (NSA), soon to be equipped with the biggest bank of computers in the world.

In Berlin, the front line of the Cold War, the CIA in 1954 dug a long tunnel under the Soviet sector to tap telephone cables. The KGB let it go on for 11 months -- its spy George Blake had warned them of the tunnel's existence. Mossad, Israeli intelligence, obtained a copy of Khrushchev's 20th Party Congress speech of 1956, denouncing Stalin's crimes, and passed it to the CIA, which broadcast it behind the Iron Curtain. This helped provoke risings in Poland and Hungary later in 1956 and general turmoil elsewhere in the communist camp.

As Khrushchev threatened and blustered, Oleg Penkovsky, an active agent, briefed the West repeatedly on the true state of Soviet military preparedness. Eisenhower and Kennedy had the benefit of his advice and knew that Khrushchev was bluffing. Penkovsky was accidentally spotted by the KGB after meeting a British contact in Moscow and was executed by a bullet in the head.

Oleg Gordievsky, of the London branch of the KGB, gave valuable service to the West for years, till he defected in 1985. He reported the Soviet Union's genuine fears of Western aggressive intentions in 1983.

Eisenhower was the first U.S. president to understand how productive aerial intelligence and satellite photography could be. In 1962 U-2 overflights revealed to Kennedy that there were Soviet missile installations in Cuba. This came as a shock to administration officials; no one had warned them Khrushchev would put them there. Nor did any U.S. agent, in Moscow or Havana, reveal another crucial fact: the Cuban army was equipped with tactical nuclear weapons. Had the United States invaded, as some urged, the use of these weapons would have come as a total surprise, and might well have triggered all-out nuclear war.

The United States had aerial photographs of Soviet tanks massing against Czechoslovakia in 1968 and Afghanistan in 1979. And detection by Sigint intercept and computer is supposed to have been so sophisticated as to enable Washington to overhear conversations of Politburo members. The real penetration of the Soviet Union was made possible by Sigint and satellites, not by agents. The KH 11 satellite was crucial to arms-control implementation; it made possible real-time verification by satellite to back up SALT II.

The crucial thing in war is to understand your opponent's thinking and intention. Toward the end of the Cold War, statesmen came to believe that there were few better ways to do this than to meet and talk to their antagonists. In the most striking reversal of all, both the United States and the U.S.S.R. came to understand that their safety depended on openness and transparency rather than secrecy. Arms-limitation agreements could only be meaningful if verified, and could only be verified if each side laid itself open to the other's inspection.

Aldrich Ames, the ex-middle-ranking CIA officer now serving life in prison, spied for money. He was paid $2.7 million, and had $1.9 million coming to him when arrested in 1994. During nearly 10 years on the KGB payroll, Ames fingered 25 CIA agents in the Soviet Union, of whom 10 were shot. One of them was Dmitri Polyakov, the ClA's most productive Russian employee, who had been recruited in 1961. A grandfather, he had retired to his dacha when the KGB came for him in 1986. A long search for the mole who betrayed him led the CIA to Ames. Markus Wolf, of East Germany's secret service, denies that spying is romantic: "It is dirty; people suffer."

Jeremy Isaacs is the executive producer of COLD WAR. Taylor Downing is a COLD WAR series producer and author of "Civil War." This essay appears in their book "COLD WAR: An Illustrated History," a companion to the television documentary, published by Little, Brown and Company.