East-West tension defined the Cold War, but its legacy is the victory of hope over fear
The bomb. The Soviet atomic bomb also alarmed the West in 1949, but its effect over the long run was to make war with the United States not more likely but less so. The single most important characteristic of the Cold War--the reason we attach the adjective to the noun--is that it went on for so long with such high levels of tension without ever producing a direct military clash between its major antagonists. The obvious explanation is nuclear weapons, which expanded the potential arena of military conflict to such an extent that the superpowers had no way of fighting each other with any assurance of keeping their own territories insulated from the resulting violence. Since wars had mostly arisen, in the past, over the protection of territory, this was a fundamental change in the way nations thought about, and used, military force. The Cold War may well be remembered as the point at which the costs of hot wars, at least among the great powers, became too exorbitant, the benefits too problematic, and the issues that had always before provoked such wars too insignificant. The fact that the Soviet Union collapsed with its military power intact is as eloquent an indication as one might want of such power's ultimate irrelevance.
Espionage. Even the spies look less sinister now than they did in 1949, despite the fact that we now know there were more of them than anyone then suspected. For the Cold War also changed our thinking about secrecy. Whereas the idea in the past had been to conceal information from enemies, a paradoxical side effect of ICBMs was the reconnaissance satellite, from which very little could be concealed. The Americans and the Russians soon saw the benefits of this new technology and agreed tacitly to tolerate it: Neither side made any effort to shoot down such spies in space, as a well-known spy in the sky, U-2 pilot Francis Gary Powers, had been shot down over the Soviet Union in 1960. The strategic arms limitation agreements of the 1970s could hardly have worked without overhead surveillance. But if transparency made sense when it came to the arms race, might it have at earlier stages of the Cold War? We know little, as yet, about how Stalin used the information his spies provided. There is reason to suspect, though, that on balance it reassured him by minimizing the possibility of surprise, just as reconnaissance satellites did for a subsequent generation of Cold War leaders. If that is the case, then some future historian may well revise what we think of the spies as well, finding that even in those deep fears there was some hope.
The world has spent the past half-century having its worst fears not confirmed. That is a big difference from the way in which it spent the first half of this century, when the opposite happened. No one could have anticipated, in 1900, that the next five decades would see unprecedented violence, including two world wars, a nearly successful effort to wipe out an entire people, and the invention of the most lethal form of military technology in human history.
But it is equally the case that few people at the beginning of 1950 could have imagined that the five decades to follow would witness great-power peace--that, although the world was hardly free from violence and injustice during the second half of the 20th century, the record was decidedly preferable to that of the first half. Fears did become hopes, although it took us a while to begin to realize what was happening. With the dancing feet, and then the hammers and crowbars, and then the bulldozers and backhoes at the Berlin Wall, however, a certain amount of progress in human affairs became difficult to deny.