AS 1945 began, the United States was phasing out of war and into peace, a peace earned by defeating fascist aggression in Europe and the Far East.
As 1945 ended, the United States was phasing out of peace and into war, a war of nerves against a former ally that had aggressive designs of its own.
Nineteen forty-five was a watershed year globally because it marked the end of World War II and the start of the cold war between the US and the Soviet Union. Events beginning in 1945 also marked a turning point for Americans: Their historic yearning to be free from foreign entanglements could no longer be indulged.
It ''was the first year in their history that Americans had a vision of world leadership and the power to act on that vision - when the US moved from being a regional power in the Western Hemisphere to being a global power,'' says Thomas Paterson, a University of Connecticut diplomatic historian.
As the end of World War II neared, President Franklin Roosevelt was so sure that a return to prewar normalcy was imminent that he predicted all US troops would be out of Europe within two years.
Instead, the six-year conflict that cost $4 trillion and 40 million lives created circumstances that forced the US to remain indefinitely at the center of world politics.
One such circumstance was the demand to construct a strong United Nations, which US policymakers saw as the antidote to the alliance systems, arms races, and power balances that had led to two world wars in the 20th century alone. Paradoxically, the cold war produced its own, far more lethal arms race that kept new alliances on a hair trigger for four decades.
The US was also compelled to remain active in the international arena by the need to design and implement a new trading system that would open the world to US products and investment.
As the world's richest nation, America was also forced to respond to the huge demand to provide aid to nations savaged by the war, and to the millions of people displaced by it. Reconstruction aid began flowing in late 1945 and culminated in the Marshall Plan in 1948, which helped underwrite the economic renaissance of Europe.
The main force pushing the US onto the world stage was the emerging threat of Soviet expansionism, which as early as the last months of 1945 was confronting the Truman administration and Congress with a difficult choice. As the Monitor's Richard Strout explained at the time: ''There is the question of whether America is going to shake a fist at Russia while demilitarizing herself.''
By the end of 1945, strains with Russia on two fronts were providing unmistakable hints that cherished dreams of disarming and returning to the isolationism of the prewar years were about to be overtaken by events.
On the Eastern front, China was the focal point of US-Soviet rivalry.
At war's end, both Washington and Moscow were nominally committed to help the Chinese nationalist government of Chiang Kai-shek regain control of territory liberated from Japanese control.
The US provided military, logistical, and financial support to the corruption-riddled Chiang government. But the Soviet Union provided clandestine aid to Chiang's rival, communist leader Mao Tse-tung, secretly turning over to Mao ''abandoned'' Japanese arms from areas occupied by Soviet troops under the terms of the February 1945 Yalta agreement.
Determined to bring an end to the civil war, President Truman, on Dec. 15, 1945, dispatched retired Army Chief of Staff George Marshall to broker a truce between the warring factions and to lay the groundwork, in Mr. Truman's words, for a ''strong, united, and democratic China.'' One year later General Marshall returned home, his mission a failure.
By 1948 the Chinese Communists had overrun Manchuria and pushed into central China. A year later, Chiang withdrew his tattered remnants to Taiwan, leaving half a billion Chinese under Communist rule, US-Soviet relations seriously strained, and politicians in Washington locked in partisan debate over ''Who lost China?''
ON the Western front, signs of discord between Washington and Moscow became evident even sooner. Even before Roosevelt's death, in April 1945, there were worrisome indications that Moscow would not follow through on its pledge, made at the Yalta Conference two months before, to support the election of free governments in the central European nations liberated from Nazi rule by the Red Army.
Tensions grew as the Council of Foreign Ministers, at a series of meetings beginning in London in September 1945, wrangled acrimoniously over the details of peace treaties with the lesser Axis powers, including Italy.
''If you don't cut out all this stalling and let us get down to work, I am going to pull an atomic bomb out of my hip pocket and let you have it,'' an exasperated, less-than-discrete Secretary of State James Byrnes told his Soviet counterpart at a low moment during the London conference.
Tensions were also exacerbated by conflicting views of how to deal with postwar Germany, where the victorious Allied powers staked out zones of occupation.
Despite an initial inclination to impose a punitive peace on Germany, US officials concluded that an industrialized, democratized Germany tied to the West would be the best guarantee against a resurgence of German militarism and the best buffer against the spread of communism into western Europe.
But burned by successive German invasions, Moscow would have none of it, even when the other Allies offered security guarantees.
''The Soviet authorities - along with the Czechs and Poles - had not forgotten Munich and they lapsed into mistrust of the Western will to keep Germany down,'' noted Herbert Feis, referring to the 1938 agreement in which Western leaders acquiesced to German aggression in Czechoslovakia.
In the end, the three Western powers - the US, France, and Britain - merged their occupation zones and the Soviets turned eastern Germany into a satellite state, shattering the last hopes for postwar cooperation in Europe and producing the division of Germany that lasted until 1989.
By the end of 1945, the US was on the verge of its first direct confrontation with the Soviet Union, over Moscow's determination to maintain troops and install a puppet government in the northern Iranian province of Azerbaijan.
A year before, British Prime Minister Winston Churchill had warned Roosevelt that Azerbaijan would provide a ''test case'' of postwar Soviet relations with the West.
In April 1946, Moscow backed down after Truman dispatched the Sixth Fleet to the eastern Mediterranean. But the incident was a harbinger of more serious confrontations to come, from Berlin to Cuba. It was the most visible sign yet that the respite from international tension earned on the battlefields of World War II was destined to be short-lived.