In diplomacy, threats that act as a deterrent have most often come in military form and have therefore implied the capability to project military power. Possessing a powerful navy gave Britain such a capability, but for much of its early history, the United States was not able to project military power and therefore made threats rarely and with questionable success. One notable deterrent effort was President James Monroe's unilateral declaration on 2 December 1823 of the independence of the Western Hemisphere, issued in order to deter Spanish intervention in Latin America and Russian expansion on to the American continent. Monroe's threat was twofold. First, he implied local military resistance if Spain tried to reestablish its colonies in Latin America or Russia expanded onto the American Northwest Coast. Second, he implied that if the European powers chose to interfere in the affairs of the Western Hemisphere then the United States would be forced to revoke its longstanding tradition of non-interference in European affairs. Ultimately, Spain did not try to reestablish its colonies, although this probably had less to do with Monroe's threats than it did with similar threats issued by Britain.
The advent of strategic bombing in the early twentieth century vastly increased the ability to project military force, and consequently led to the transformation of deterrence into its modern form. The twin technological developments of high explosives and aircraft that could deliver them to their targets made strategic bombing a decisive factor in modern warfare. It was first used in limited form in World War I in Germany's zeppelin (airship) attacks against Britain. German scientists had not only developed a way to incorporate poison gas into a bomb, thereby creating the first type of the weapons now classed as weapons of mass destruction, but had also developed a way to float zeppelins over enemy lines and drop their payloads on British cities. Although not widely recognized at the time, this ability to move beyond the confines of the battlefield and to attack an enemy's cities directly revolutionized modern warfare as offensive strategic bombing threatened to break the defensive deadlock that had evolved from tanks, machine guns, and trench warfare.
At the same time it introduced a new factor that, although intangible, was no less powerful. Like the German V-1 and V-2 rocket strikes during World War II, the German zeppelin of World War I resulted in few deaths, but the potential that the technology seemed to hold for extending the battlefront from the trenches to civilian homes captured the British public's imagination to an extent that far outweighed objective casualty counts. For the first time a military weapon was used not only for the tactical calculations of policymakers, but also to strike terror into the home front, which had become an increasingly vital component of modern warfare. The fluid variable of civilian morale suddenly became as important as military morale as the civilian population reacted to the new threat to their homes and lives. Moreover, during World War II, as strategic bombing became vastly more effective (and deadly), the industrial and economic hearts of an enemy became additional viable targets. Allied long-range bombers all but destroyed the German industrial city of Dresden, while the American firebombing of Tokyo started fires that raged out of control for days at a time. With the development of the atomic bomb, a weapon blatantly unable to discriminate between military and civilian targets, strategic bombing was taken to its extreme.
The development of the atomic bomb was the culmination of the top secret Manhattan Project, an extraordinary collaboration of international scientists headed by J. Robert Oppenheimer that was backed by vast resources provided by the U.S. government. Stringent security precluded public debate about what role the new weapon would have, but among those few who had information about the project's overall objective and progress, there was growing awareness that the new weapon would be unlike anything that had come before; it would perhaps even create "a new relationship of man to the universe," as a committee chaired by Secretary of War Henry L. Stimson put it. Such an unconventional weapon clearly required unconventional thinking. As Oppenheimer recognized, the elements of surprise and terror were as intrinsic to it as fissionable nuclei. In a top-secret report submitted to the War Department on 11 June 1945, a small committee chaired by physicist James Franck suggested that the psychological impact of the explosion might be more valuable to U.S. military objectives than the immediate physical destruction. In the hope that a demonstration of the destructive potential of the atomic bomb might be enough to compel the Japanese to surrender, the Franck Committee proposed a public demonstration in an uninhabited region.
After considering the various proposals, President Harry S. Truman concluded that a demonstration in an uninhabited region would likely be ineffective, and therefore ordered that the bomb be used against Japanese cities, a decision that has been passionately debated ever since. Some have argued that Truman's motivation was less the war in the Pacific than the impending contest with the Soviet Union and that as such it represented the opening gambit of so-called "atomic diplomacy," while others argue that the decision was not only militarily sound but necessary, and that it ultimately saved hundreds of thousands of American and Japanese lives. But whatever Truman's motives, at 8:15 A.M . on 6 August 1945, an American B-29 Superfortress long-range bomber named the Enola Gay delivered its single atomic bomb to the target of Hiroshima, the second most important military-industrial center in Japan. Upward of seventy thousand people were killed instantly in the blast. Three days later another bomb was dropped on Nagasaki, killing at least twenty thousand. In the following weeks the death counts in both cities rose as the populations succumbed to radiationrelated illnesses.
As the wire services flashed the story around the globe, journalists who had witnessed the Trinity test blast at Alamogordo, New Mexico, three weeks earlier on 16 July, were now free to write about what they had seen and help a startled world comprehend what had happened. The American public's reaction was a mixture of relief that the end of the war was in sight, satisfaction that revenge had been exacted upon the perpetrators of the Pearl Harbor attack, and a sober recognition of the responsibility the new weapon carried with it. In strategic terms, there was not yet such a thing as a U.S. atomic stockpile, despite President Truman's implication in his press statement announcing the Hiroshima bombing that atomic bombs were rolling off the production line. Had the first two atomic bombs failed to bring a Japanese surrender, some time would have passed before more were ready. Within days of the Nagasaki bombing, however, the Japanese leaders finally succumbed to the inevitable and formally surrendered to General Douglas MacArthur's forces on 2 September 1945. From that moment, the priority for U.S. military forces was not building more bombs, but going home.
As the United States demobilized in the postwar period, relations with the Soviet Union deteriorated. The coincidence of the beginning of the Cold War and the dawn of the nuclear age ensured that the history of the two would become inextricably entwined. During the early Cold War, the primary strategic contest was for Europe, and Germany in particular, but throughout the continent evidence mounted of a clash of interests and ideology. Within a few short years of the end of World War II, the U.S. government had publicly identified the Soviet Union as its primary strategic threat. And if war did break out in Europe, the Soviets had vastly more conventional forces and the geographical advantage as well. The challenge for U.S. defense planners, therefore, was to find a way to project the U.S. atomic force. Nevertheless, the planners moved slowly to devise a coherent nuclear strategy to serve foreign policy interests. Although fully recognizing that the Soviets would sooner or later develop the atomic bomb, American policymakers struggled to find a way to take advantage of the atomic monopoly. However, the American population was still weary from World War II and constrained military budgets were shrinking, so atomic development was a low priority. It was not until Dwight D. Eisenhower became president that a coherent deterrent role was found for the U.S. nuclear arsenal. But by the end of the Truman administration, defense budgets were growing rapidly, and that administration made some effort to bring military policy into the atomic age. Having recently witnessed how much sacrifice Soviet leader Joseph Stalin was willing to impose on his countrymen in the defense of the USSR, it was clear to U.S. policymakers that if war should break out between the two superpowers, the small stockpile of American atomic bombs that had been built up since Nagasaki would not guarantee victory.
In use, the atomic bomb was an offensive weapon. For the American atomic monopoly to be cast in a defensive role, that role had to be to prevent war altogether through the very threat of retaliation. Bernard Brodie, one of the first defense intellectuals to engage publicly the implications of the atomic bomb, succinctly summarized the momentous shift in military affairs that the bomb had sparked. "Thus far the chief purpose of our military establishment has been to win wars," Brodie commented in The Absolute Weapon (1946). "From now on its chief purpose must be to avert them." To that end, and recognizing that atomic weapons did not fall easily under the existing military force structure, the Truman administration in March 1946 created the Strategic Air Command (SAC) headed by General George Kenny. Adopting the motto "Peace is our profession," SAC's mission was to give the United States a long-term capability to project U.S. nuclear force anywhere on the globe. SAC's existence exemplified the paradox of deterrence strategies as summarized in the Latin adage Qui desiderat pacem, praeparet bellum (Let him who desires peace, prepare for war). Unlike the case with conventional military forces, for the remainder of its existence SAC's success would be measured not by its performance in battle, but by its never having actually to engage in combat.
The Cold War contest was ultimately a strategic one, but it was more often manifested in a series of short-term political contests. Committed to a policy of containing communist expansion, the Truman administration found itself having to rethink its assumption that atomic weapons would deter simply because they existed. By the end of 1948 there was mounting evidence that the American atomic monopoly was having little success in deterring communist political expansion through Europe; the threat of communist subversion in Greece and Turkey in 1947, the communist coup in Czechoslovakia in February 1948, and the strong communist presence during the Italian election of April 1948 all seemed to provide evidence to that effect. And in the first truly nuclear crisis of the Cold War, the Berlin blockade of 1948–1949, the Truman administration could manage only a half-hearted atomic threat that, if Stalin had pushed the matter, would likely have been revealed as a bluff. In a move with the twin objectives of temporarily bolstering the flagging British strategic bombing force and sending an atomic threat to Stalin, American B-29s were deployed in Britain at the height of the Berlin blockade crisis. But this early attempt at nuclear coercion was unconvincing. The B-29s initially sent were not modified to carry atomic bombs; furthermore, it was public knowledge that there was no procedure in place to store atomic warheads overseas. By mid-1948 even Secretary of Defense James Forrestal had to admit that American military planning, including its nuclear strategy, was "patchwork" at best. If America's national security was to realize the full potential of the atomic bomb for deterrence, a thorough rethinking would be needed.
One solution preferred by many was for the United States to exploit the window of opportunity by launching a preemptive strike against the Soviet Union. Former British prime minister Winston Churchill suggested sending Stalin an ultimatum stating that if he did not desist from his expansionist policies, U.S. planes would use atomic bombs against Soviet cities. The U.S. commander in Germany, General Lucius Clay, agreed. Other military voices in Washington lamented the wasting of an opportunity. The calls became more urgent as that window of opportunity seemed to be closing. When the Soviet Union detonated its first atomic device in August 1949, it caught the West by surprise. The mastering of the atomic process by Soviet scientists was not unexpected in a general sense, but beyond the inner sanctum of intelligence officials and defense planners, the Soviet achievement was never seriously anticipated beyond the vaguest of timetables. To make matters more alarming, two months later, in October 1949, Mao Zedong's Communist Party emerged victorious in China's civil war, a development seen in Washington as proof that Moscow's ambitions were not confined to Europe but were global.