The available evidence now suggests to historians that Julius Rosenberg did in fact spy for the Soviet Union. The evidence against Ethel Rosenberg, however, is considered flimsy at best. But whatever they may have done, it is far from evident that they had handed Moscow the key to its first atomic bomb, as charged at the time.Here's the Wikipedia page:
The couple remain a special case. The United States has had many spies over the last 50 years, including some believed to have done great harm to American interests. Not one was put to death.
The Rosenbergs were the only two American civilians to be executed for espionage-related activity during the Cold War. In imposing the death penalty, [Judge Irving] Kaufman noted that he held them responsible not only for espionage but also for the deaths of the Korean War:
“I consider your crime worse than murder...Commenting on the sentence given to them, Julius Rosenberg claimed the case is a political frame-up.
I believe your conduct in putting into the hands of the Russians the A-Bomb years before our best scientists predicted Russia would perfect the bomb has already caused, in my opinion, the Communist aggression in Korea, with the resultant casualties exceeding 50,000 and who knows but that millions more of innocent people may pay the price of your treason. Indeed, by your betrayal you undoubtedly have altered the course of history to the disadvantage of our country. No one can say that we do not live in a constant state of tension. We have evidence of your treachery all around us every day for the civilian defense activities throughout the nation are aimed at preparing us for an atom bomb attack.”
“This death sentence is not surprising. It had to be. There had to be a Rosenberg case, because there had to be an intensification of the hysteria in America to make the Korean War acceptable to the American people. There had to be hysteria and a fear sent through America in order to get increased war budgets. And there had to be a dagger thrust in the heart of the left to tell them that you are no longer gonna get five years for a Smith Act prosecution or one year for contempt of court, but we’re gonna kill ya! ”...Marxist Nobel Prize-winning existentialist philosopher and writer Jean-Paul Sartre called the trial "a legal lynching which smears with blood a whole nation. By killing the Rosenbergs, you have quite simply tried to halt the progress of science by human sacrifice. Magic, witch-hunts, autos-da-fé, sacrifices – we are here getting to the point: your country is sick with fear ... you are afraid of the shadow of your own bomb." Others, including non-Communists such as Jean Cocteau, Albert Einstein and Nobel-Prize-winning physical chemist Harold Urey, as well as Communists or left-leaning artists such as Nelson Algren, Bertolt Brecht, Dashiell Hammett, Frida Kahlo and Diego Rivera, protested the position of the American government in what the French termed America's Dreyfus affair. In May 1951, Pablo Picasso wrote for the communist French newspaper L’Humanité, "The hours count. The minutes count. Do not let this crime against humanity take place." The all-black labor union International Longshoremen’s Association Local 968 stopped working for a day in protest. Cinema artists such as Fritz Lang registered their protest. Pope Pius XII appealed to President Dwight D. Eisenhower to spare the couple, but Eisenhower refused on February 11, 1953, and all other appeals were also unsuccessful.
Senator Daniel Patrick Moynihan, vice-chairman of the Senate Select Committee on Intelligence, investigated how much the Soviet spy ring helped the USSR to build their bomb. In 1945, Moynihan found, physicist Hans Bethe estimated that the Soviets would be able to build their own bomb in five years. “Thanks to information provided by their agents,” Moynihan concluded in his book Secrecy, they did it in four. That was the edge that espionage gave them: one year.”