The Korean War produced neither a glorious victory, as World War II did, nor an ignominious, Vietnam-like defeat. Except at the outset, it was not an especially popular war, but neither did it produce the harsh divisions that Vietnam would bring less than a generation later.

An inconclusive, ill-understood conflict, the Korean War, which broke out 40 years ago Monday, quickly faded in popular memory as Americans eagerly went on what one historian called the "American High" of 1950s prosperity, domesticity and world power. Korea became America's forgotten war.

Yet the conflict profoundly altered the United States -- its politics, its understanding of the powers of the presidency, its military position and its world role.

It ratified America's standing as a global power, but also prepared the way for Vietnam. Among the conflicts in America's history, the forgotten war had some of the most far-reaching consequences.

"Korea," said Richard J. Barnet, co-director of the left-of-center Institute for Policy Studies, "will be seen in retrospect as even more important than Vietnam." This view, though much debated, has gained ground among historians.

The facts and figures alone point to the enormous changes Korea helped bring about. In 1949, military spending stood at $14 billion. By 1953, it had risen to $44 billion. In June 1950, the United States had 590,000 men and women in the military. By the time of the Korean armistice in 1953, the figure was 3.6 million.

Paul H. Nitze, director of the State Department's policy planning staff during Harry S. Truman's presidency, noted that the military buildup had been called for in an April 1950 National Security Council report.

But the invasion of South Korea by the Communists from the North gave supporters of the buildup the evidence they needed to win approval for what became the largest military spending program in history.

"This was clear proof that there was a military threat," Nitze said in an interview.

William L. O'Neill, a historian at Rutgers University and author of "American High," a book on the 1950s, said America's response to Korea represented a fundamental alteration of the "containment" approach toward the Soviet Union first put forward after World War II by George F. Kennan, the influential diplomat whose views helped shaped American global strategy.

"Kennan's definition of containment was political and economic," O'Neill said. "Korea represented the militarization of the Cold War."

Although the United States had not included South Korea as part of its "defensive perimeter" in Asia, the Truman administration, under assault for "losing China" to the Communists in 1949, was hardening the American approach to the Soviets. Korea presented the nation with an opportunity -- and, in the eyes of the administration, a responsibility -- to take a stand.

The Korean War started at 4 a.m. on a rainy Sunday June 25, 1950 -- Saturday afternoon, the 24th, in Washington. After a two-hour artillery barrage, 90,000 North Korean troops poured across the 38th Parallel that divided the Korean peninsula and began driving south.

Nitze remembers the day well: He was fishing for salmon on the Upsalquith River in New Brunswick when he learned of the invasion from a guide who heard of it on the radio. "The next morning, I hopped in my canoe, raced down the river," and eventually made his way to Washington.

Things there were moving quickly, since the administration was convinced that Soviet dictator Joseph Stalin was behind the attack. In "God's Country: America in the Fifties," historian J. Ronald Oakley quotes a State Department official comparing the relationship between the Soviets and North Korea to that "between Walt Disney and Donald Duck."

That Sunday afternoon, the U.N. Security Council, boycotted by the Soviets for the U.N.'s refusal to seat Communist China, passed a U.S.-sponsored resolution condemning the attack. Two days later, the United Nations called on members to assist South Korea. Ultimately, while most of the air and naval forces that served in Korea were American, half the ground forces came from other nations. Truman turned to Gen. Douglas MacArthur, the 70-year-old World War II hero and commander of the American occupation troops in Japan, to command U.N. forces.

If Truman felt a need for U.N. blessing, he did not seek a congressional declaration of war, setting a critical Cold War precedent. Congress acceded to this shift in the balance of power between the executive and legislative branches that became so controversial during the Vietnam War.

Korea also hastened a profound change in the Republican Party: the weakening and ultimate collapse of the party's isolationist wing that found leadership under Ohio's Sen. Robert A. Taft.

Taft's initial reaction to the North Korean invasion was characteristic. He asserted that the country "shouldn't get stampeded into war." But within days, the conservative Republicans, long more interested in Asia than Europe, fell into line. Anticommunism, not isolationism, became the hallmark of conservative foreign policy.

Sen. Joseph McCarthy (R-Wis.) also had something to do with that shift, and Korea strengthened his hand. McCarthy had made his famous Wheeling, W.Va., speech charging that 205 Communists were working in the State Department on Feb. 9, 1950. Four months later, the United States was in a shooting war with Communists and what little tolerance existed for domestic communism dissipated further.

As the war dragged on, McCarthy's charges of criminal incompetence in high places won an ever-wider hearing. "McCarthyism was born of a sense of frustration and suspicion," said John Patrick Diggins, a historian at the University of California at Irvine. "It was the psychological and political counterpart to our frustration over the lack of achievement in the war itself."

The first few weeks of the war were disastrous and by August, U.N. forces had been driven to a tiny corner of South Korea. But in a stroke whose daring lives on in military history, MacArthur launched an amphibious landing on the port of Inchon, far behind North Korean lines, on Sept. 15. By late September, the North Koreans were driven back across the 38th Parallel.

Had the war stopped then, the United States could have claimed a quick victory. But the administration, urged by MacArthur, ignored threats from the Chinese Communists that they would intervene and continued to drive north.

By November, the allies controlled most of North Korea and were approaching the Yalu River, the border with China. Then about 200,000 Chinese troops crossed the border and overwhelmed the allies, pushing them well south of the 38th Parallel.

The debacle played into a feud between European-oriented generals and "Asia-first" generals such as MacArthur. MacArthur fought back, complaining publicly about the foolishness of fighting a limited war. Coining a phrase that became a conservative battle cry for the next 40 years, MacArthur declared that "there is no substitute for victory" and called for all-out war on China. On April 11, 1951, Truman fired MacArthur for insubordination, unleashing a wave of popular outrage.

The war became a murderous grind as the allies slowly and at great cost pushed the Chinese back, with the battle lines eventually settling just north of the old division line.

Dwight D. Eisenhower, elected in 1952 on a promise to end the war, ratified this ambiguous conclusion with the armistice of July 27, 1953. By then, 33,629 Americans had died in battle and 103,000 had been wounded.

"We have won an armistice on a single battleground, not peace in the world," Eisenhower declared, capturing the sense that the Korean War marked just the beginning of a much longer Cold War that would be played out again later in Vietnam. "We may not now relax our guard nor cease our quest."