The South Korean capital of Seoul fell on June 28 and the North Koreans continued to advance south, finally repulsed in September when General Douglas MacArthur staged a bold amphibious landing behind enemy lines at Inchon. But MacArthur, officially commanding UN forces, pressed north across the 38th parallel into North Korea itself, approaching the Yalu River frontier with China. This triggered a Chinese invasion, which drove American-led forces all the way south back over the 38th parallel to Seoul, which Chinese and North Korean forces recaptured in January 1951, before UN forces again liberated it in March.

In April, Truman relieved MacArthur of command for refusing to fight a limited war. A limited war, in which American-led forces did not chase the North Koreans back north across the 38th parallel, was clearly in the spirit of containment. But MacArthur’s outsized ego had intervened, something that often happens in history and which Shakespeare’s historical dramas are replete with. Which is why history, including the Cold War, often does not go according to some preconceived plan or grand strategy. As it would turn out, now that the Chinese were deeply involved in the fighting thanks to MacArthur’s original misjudgment, hostilities continued for another two years before an armistice was concluded, with the 38th parallel once again set as the border between the two Koreas. Little had been accomplished except for over 33,000 American dead. True, the spread of communism had been halted, but not peacefully, as the original theory of containment implicitly hoped for.

CONTAINMENT CONSTITUTED an expectation that war could be avoided. And an all-out nuclear war would be avoided for over four decades. But the United States would still have to cope with middle-sized wars that carried a steep blood cost.

It was under President Eisenhower that the armistice was concluded. Eisenhower had vowed during his 1952 campaign against Illinois governor Adlai Stevenson II that, “I shall go to Korea,” in order to end the inconclusive war that Truman had become burdened with. Eisenhower biographer Jim Newton of the Los Angeles Times observes that when Eisenhower made his trip to the Korean Peninsula after being elected president (but before his inauguration), none of the big questions about the Cold War in Asia had been settled. “Was the enemy North Korea, China, or the Soviet Union? Was America’s goal to repel [a] North Korean invasion, to unify Korea under democratic rule, or to topple Chinese Communism?” There was no agreement on the answers and little grand strategy to draw upon, especially since Communist rule in China was only a few years old then, and thus still seen as quite vulnerable.

Eisenhower, as it happened, settled for the armistice that would give neither side a victory. It was a clue to how he—arguably much more than Kennan—decided the trajectory for the coming decades of the Cold War. Eisenhower’s “middle way,” an organic outgrowth of his character and personality, rejected the advice of generals who believed that in this early phase of the conflict both the Soviet Union and China could be militarily defeated. Eisenhower also rejected the advice of those who believed that the likes of Nikita Khrushchev, Mao Zedong, and Fidel Castro were genuinely sincere in their calls for international peace. Kennan’s containment theory was in the back of Eisenhower’s mind, but it was a mind that was driven by an innate caution regardless.

In the mid-1950s and again in the late-1950s, Eisenhower nearly committed to launching nuclear attacks on China over the fate of Quemoy and Matsu, islands in the Taiwan Strait held by the Nationalists and claimed by the Communists. Eisenhower, in the end, would hold back from using nuclear weapons several times: when they might have ended the Korean War in circumstances more favorable to the United States, when they might have saved the French garrison at Dien Bien Phu in Vietnam in 1954, and when they might have quelled Soviet threats to Berlin. Again and again, his advisers, who clearly were not motivated by containment as it is today understood, urged Eisenhower to use his nuclear arsenal. But Eisenhower always hesitated. In his 1953 inaugural address, he had committed to both security and prudence, and not to forcing America’s values on others. He even refrained from threatening force to defend others who were demanding their own freedom: witness the Hungarian Revolution in 1956. The 1950s were at once extremely dangerous and tumultuous. They are remembered with nostalgia as a safely dull decade only because the true genius of leadership is not to make good things happen, but to prevent bad things from happening. On the whole, as the Eisenhower presidency makes clear, Kennan would be lucky in his Cold War presidents.

But Kennan, it must be said, also helped his own luck. In 1953, at the beginning of his presidency, Eisenhower convened a large group of advisers to devise a new Cold War strategy. Known as Project Solarium, because it began in the Solarium Room of the White House, it came up with Eisenhower’s “New Look” approach to the Cold War. New Look saw the struggle with the Soviet Union and China as long and practically unending, so that Eisenhower committed to aggressively rolling back Communism whenever possible. Nevertheless, containment would still influence the sensibility of Eisenhower’s approach as events unfolded, mainly because Kennan was one of the key advisers for Project Solarium. The Long Telegram and Mr. X’s “The Sources of Soviet Conduct” required help from not only wise presidents, but also from Kennan himself, as he continued to make his voice heard.

Kennan also got help from thermonuclear bombs. Eisenhower was the first president to have an arsenal of atomic bombs and not use them. This was not fated in advance. If America had another somewhat more impulsive president at the time—and we have had such—Hiroshima and Nagasaki may not have constituted the only places to suffer nuclear destruction. Furthermore, it was during Eisenhower’s administration that both superpowers deployed hydrogen bombs in vast numbers. A direct military confrontation between the United States and the Soviet Union suddenly became much harder to contemplate, for fear it might escalate into a nuclear exchange. Therefore, the negative reasons on which containment originally rested became much more compelling: since a U.S.-Soviet war was becoming unthinkable and outright isolationism in a nuclear age was simply irresponsible.

The most dangerous chapter of the Cold War would turn out to be the first, between Stalin’s Bolshoi Theater speech and the Cuban Missile Crisis, by which time John F. Kennedy was president. After the United States learned in October 1962 that the Soviets had transferred nuclear warheads atop missiles to Cuba, ninety miles from Florida, Kennedy ordered a naval blockade of Cuba. Neither side budged as one day followed another, both sides contemplating that the next step by either of them might actually be the use of nuclear weapons. The Soviets finally withdrew the missiles from Cuba in November, while the United States secretly agreed to withdraw nuclear warheads from Turkey that had been aimed at the Soviet Union.

During those fateful days in October, the two superpowers stared into the abyss for the first time, and neither side liked what it saw. Thermonuclear annihilation had journeyed from the somewhat abstract to the real and vivid. In the wake of the Cuban Missile Crisis, both sides agreed to establish a “hotline” to communicate more directly with each other. This was followed by a nuclear test ban treaty and other measures, including regular summit meetings, that taken together both ordered and set parameters to the Cold War. The Cold War went on still, but now it had rules that it hadn’t had before. The world, thanks to the nerve and the restraint of both Eisenhower and Kennedy, had become a little less dangerous. Containment was part of this recipe, but only a part.

CONTAINMENT WAS ambiguous when it came to Southeast Asia. Kennan’s theory envisioned the United States holding the line against communist advances in the third world, even as Kennan himself was skeptical of the critical importance of Vietnam to U.S. strategy. And later, in pursuing actual war in Vietnam, Kennan, according to Gaddis, believed that the United States “appeared to be sacrificing its own best traditions of constitutional and moral responsibility.” Still, we are talking here about the history of a grand strategy, as encased in the Long Telegram and the X article, more than we are talking about Kennan’s various views of U.S. foreign policy as they evolved over the decades. The fact is, communism seemed to be on the march in Indochina in the 1950s and early 1960s, and containment as elucidated as a grand strategy in 1946 and 1947, at the very beginning of the Cold War, did not answer the questions about when, how, and at what level—if at all—the United States should intervene.

Little is fated to happen. In this sense, the assassination of John F. Kennedy, like the assassination of Abraham Lincoln, which probably delayed the achievement of civil rights in the South for decades, was a tragic event. Kennedy, who was a genuine war hero and scion of a high-powered, supremely arrogant, and rather wealthy family, even as he had been rattled by the fiasco of the Bay of Pigs and hardened by the Cuban Missile Crisis, was a man not about to be intimidated by the Pentagon brass and its civilian leadership, especially after the U.S.-inspired coup against South Vietnamese president Ngo Dinh Diem in early November 1963 had ended so bloodily. It is hard to believe that the Vietnam War would have evolved in the way that it did, were Kennedy president and not Lyndon B. Johnson—a foreign policy neophyte who gave the Pentagon hawks little pushback.