In 2001, Jay Winik scored a commercial and critical success with his best seller “April 1865,” a vivid account of the final month of the Civil War, which Winik convincingly argued was a crucial turning point in American history. Robert E. Lee and Ulys­ses S. Grant, among others, play major roles in “April 1865,” but Abraham Lincoln towers above all of them.

In “1944: FDR and the Year That Changed History,” Winik has turned his attention to another president, war and pivotal moment. This time, his thesis is more difficult to decipher and less persuasive. Reading “1944,” I was reminded of “Where’s Waldo?,” the classic picture book series whose brightly colored illustrations challenge young readers to find the title character, hidden in huge crowds. Similarly, the key points in “1944” are surrounded by vast amounts of information about Franklin Roosevelt and World War II, much of it irrelevant to the main story. “Why this book?” I wrote on Page 78. A dozen or so pages later, the answer began to emerge.

In the spring of 1944, as the Allies prepared for D-Day and the beginning of their final showdown with Germany, the Nazis embarked on an endgame of their own — finishing off the Jews of Europe. Having already killed some five million Jews, the Holocaust’s overseers worked overtime at Auschwitz to exterminate the last large Jewish population on the Continent — the 750,000 in Hungary.

Two months before D-Day, two young Slovak Jews escaped from Auschwitz, determined to warn Roosevelt and the other Allied leaders about “the impending massacre . . . and rally the forces of rescue and rebellion.” Winik frames their escape as a race against time. Would their eyewitness report reach the Allies before all European Jews were annihilated? Would it prompt Roosevelt to expand his focus beyond the invasion of Europe and ultimate victory — and strive to save the Jews still alive?

The president, already well aware of Auschwitz and the other Nazi extermination camps, failed to do so and thus missed his chance to claim what Winik calls his “Emancipation Proclamation moment.” In Winik’s view, Roosevelt should have emulated Lincoln, whose 1863 order freeing the slaves in Confederate states made abolition an explicit Union goal in winning the Civil War. In the same vein, Winik contends that Roosevelt should have imbued World War II with a higher moral purpose, making it not only a fight against the Axis but also “a war against the Final Solution.” He adds, “In 1944 he had his chances.”

Such arguments sidestep certain realities, beginning with the emphasis on 1944 as a potentially pivotal year for Jewish rescue. By then, the opportunity to save a sizable percentage of Jews had long since vanished. Before the war and during its first two years, Nazi Germany had been open to the idea of Jewish emigration. But the United States and Britain, among other countries, closed their borders to all but a handful of Jewish refugees, and in early 1942 the Nazis began implementing the Final Solution in death camps deep inside occupied Poland. By the time of D-Day, a vast majority of European Jews were dead. Winik argues that many thousands of Jews in Hungary could have been saved if the Allies had bombed Ausch­witz in 1944, but whether such raids could have had a significant effect is still hotly ­debated.

Unlike slavery and its abolition, which were at the heart of the Civil War, rescuing Jews in World War II was always regarded as an extraneous issue by the United States and its allies. In the 1930s and early 1940s, overt anti-Semitism was a fixture in American society. According to public opinion polls, most people opposed admitting more Jewish refugees into the United States. The State Department, rife with anti-Jewish bigotry, not only sought to keep out Jewish immigrants but also worked to suppress information about the Holocaust and blocked private efforts to save European Jews who had not yet been ensnared in the Nazi web. Although Roosevelt expressed sympathy for the Jews’ plight and promised postwar retribution for their executioners, he did little or nothing to encourage their rescue or to check the State Department’s obstructionism.

In January 1944, however, Roosevelt did an abrupt about-face. Bowing to intense pressure from elsewhere in his administration, he agreed to create a federal agency called the War Refugee Board, whose sole purpose was to save the last remnants of European Jewry. The pressure came from a group of young lawyers in the Treasury Department, one of whom had learned of the State Department’s sabotage of rescue efforts. After investigating, he and his colleagues presented an explosive memo to their boss, Treasury Secretary Henry Morgenthau Jr., titled “Report to the Secretary on the Acquiescence of This Government in the Murder of the Jews.” They urged Morgenthau to show Roosevelt their findings and demand that a government agency be formed to focus exclusively on Jewish rescue.

Morgenthau, the sole Jew in Roosevelt’s cabinet, was faced with a torturous dilemma. Having played down his Jewishness all his life, he was being asked to confront Roosevelt, whose friendship he treasured, on an issue he had worked hard to avoid. After agonizing for two days, he followed his conscience.

The only way to win Roosevelt’s agreement, he told his subordinates, was to frame the argument as a political rather than a moral necessity. In a presidential election year, the last thing Roosevelt needed was a scandal over allegations of government efforts to prevent the rescue of Jews. To bolster his case, Morgenthau could point to a recent groundswell of congressional support for a rescue agency — the result of an intensive public relations campaign sponsored by Jewish activists.

The Treasury report, Morgenthau noted, gave him the leverage to tell Roosevelt that the State Department’s obstructionism could no longer be kept secret: “It is going to pop, and you have either got to move very fast, or the Congress of the United States will do it for you.” On Jan. 16, he made exactly that point to Roosevelt, who halfheartedly defended the State Department, then gave in and signed off on the plan.

This gripping, little-known story appears late in Winik’s book, and he tells it well. Thanks to the War Refugee Board, 1944 did indeed mark a transformation in America’s response to the Holocaust, although not in the climactic fashion suggested by Winik’s subtitle. The board did not change history, nor did Roosevelt engineer its creation. Nonetheless, according to one of its agents, it “injected new life and hope into . . . refugees throughout the European continent.”

Run by the Treasury Department lawyers, the board financed operations that smuggled more than 50,000 Bulgarian and Romanian Jews to safety. Its agents also managed to keep alive thousands of Jewish children hiding in France. Although most Jews in Hungary had been deported to Auschwitz by the time the board became operational there, its emissary in Budapest, the Swedish businessman Raoul Wallenberg, managed to save tens of thousands of lives.

In all, the War Refugee Board and its operatives were credited with rescuing more than 200,000 Jews from the Holocaust — an impressive feat, to be sure, but only a tiny fraction of the millions murdered by the Nazis. The success of the board’s 11th-hour effort underscores the haunting question that runs through Winik’s book: How many more could have been saved had America acted sooner?