Called to testify before a congressional hearing in the early 1950s, Walter Bedell Smith was asked about George C. Marshall. Bedell Smith had served under Marshall as a general in World War II, when Marshall was chief of staff of the Army, and later as America’s ambassador to the Soviet Union, when Marshall was secretary of state. Bedell Smith had a fiery temperament as well as a generous degree of self-regard. Considering Marshall — or the general, as he was known the world over — Bedell Smith replied that for him to comment upon George Marshall would be like the molehill commenting upon the mountain.

It is difficult to imagine a high-ranking leader today discussing his or her senior in similar terms. Marshall was the most revered American of his time. No American has matched his stature since. During World War II, Dwight Eisenhower, George Patton, and Douglas MacArthur reported to Marshall. Dean Acheson, George Kennan, and Paul Nitze all served under him in the State Department. They are an illustrious group. A litany of memoirs survives as testimony that none was shy about asserting his place in those dramatic World War II and postwar years.

Marshall, in contrast, declined a multimillion-dollar offer to publish his memoirs. To him, self-promotion was anathema. Perhaps that helps to explain why many historians have not afforded Marshall his due, and why many Americans are more inclined to think of the subordinates that he led than the man himself, the self-effacing helmsmen of that age. Only recently have a spate of World War II and postwar histories rediscovered George Marshall, and reappraised what he meant to America during that profoundly consequential era.

Nicolaus Mills’s new “Winning the Peace: The Marshall Plan & America’s Coming of Age as a Superpower” (Wiley, 304 pages, $29.95) can be added to that list. Mr. Mills’s book is a considered and well-researched assessment of one of America’s greatest peacetime foreign policy programs. The Marshall Plan was a $13 billion assistance program to promote recovery in war-torn and postwar-weary Western Europe, but more than that, it was the centerpiece in America’s postwar grand strategy of containing the Soviet Union and bringing Western Europe into an American-led political and economic sphere.

Some historians have treated Marshall, secretary of state between 1947 and 1949 — the years of the Plan’s inception and launch — as only a figurehead of the great enterprise. Some have suggested that Marshall’s subordinates, such as Acheson and Kennan, were the real intellects behind the Plan, that Marshall did little more than lend his name to the effort. Mr. Mills sides with those who believe the general’s role was much more substantial. He argues convincingly, for example, that Marshall drew on the extraordinary currency of his prestige and congressional and national standing to see the Plan through to passage at a time when Americans and Congress were weary of a great postwar foreign commitment. He also rightly asserts that Marshall was the true author of the famed June 5, 1947, speech, given at a Harvard University commencement, which extended America’s extraordinary offer to a beleaguered Europe — as well as to the Soviet Union. (Stalin would refuse the offer, helping to codify the East-West division and thereby trigger the Cold War.) “We are remote from the scene of these troubles,” Marshall told his audience at Harvard that sunny day. “It is virtually impossible at this distance … to grasp at all the real significance of the situation. Yet the whole world’s future hangs on a proper judgment … of just what … can best be done, what must be done.”

Those assertions are important to the history of the time and Marshall’s proper place therein, but they are not original. Mr. Mills does deliver a fresh and creative insight, though, in linking Marshall’s experience with the New Deal, when he was a senior military officer stationed in various Army camps throughout the country, to some of the core beliefs and convictions that would help drive his support for the program that was to bear his name. Under President Franklin Roosevelt’s flagship New Deal program, the Civilian Conservation Corps, hundreds of thousands of erstwhile unemployed young men were put to work on various conservation projects. The War Department was given responsibility for feeding, paying, housing, and clothing them. Marshall, Mr. Mills explains — drawing in large part, it seems, from Forrest Pogue’s authoritative multi-volume biography — assumed responsibility for a good portion of this work. “I had the opportunity both to build the camps and to get into close contact with the boys,” Marshall recalled.

What he witnessed left a powerful impression: Marshall saw the young men in this program transformed by the government assistance offered, and by the chance to provide for themselves. He saw first-hand the role that a sound government intervention could play in providing security, hope, and purpose to people in dire straits. The CCC was not charity; it did not dole out money to the destitute. It was a creative, pragmatic response to a desperate time that drew on the willingness of people to help themselves and the belief that government, when necessary, had an ineluctable and essential role to play.

When Marshall looked out at the tattered European landscape in the winter and spring of 1947 he saw the looming Soviet threat; he saw emergent national Communist parties in Western Europe, and he saw dislocated economies, dysfunctional and, in many cases, nearing bankruptcy. But he also saw opportunity. He believed that if America offered a hand in partnership, an invitation for Europe to help itself — and then a sound, imaginative program to that end — Western Europe could emerge into a stable, secure, and democratic region. In the 60 years to follow, Europe blossomed into a unified, peaceful, and prosperous continent to degrees that eclipsed the expectations of the Plan’s architects.

The story of postwar Western Europe is one of the 20th century’s greatest achievements. It is, first and foremost, a European achievement. But, without American assistance — without the aid, the psychological boost that came with American capital and partnership, and the structural economic adjustments that were central to the Plan and to launching Europe on a path to integration and, eventually, economic unification — it is very possible that it would not have happened.

For that, above all others, we have the general to thank.

Mr. Behrman is a fellow at the Carr Center for Human Rights at the John F. Kennedy School of Government at Harvard University, and the author of “The Most Noble Adventure: The Marshall Plan and the Time When America Helped Save Europe.”