It is hard to believe Franklin Delano Roosevelt and Josef Stalin exchanged handwritten letters about how to win World War II. It is perhaps even harder to believe the entire exchange has never before been published. We therefore owe a debt to Susan Butler, who has brought together for the first time the entire extent run of Roosevelt’s and Stalin’s letters (Yale University Press, 361 pages, $35). It is a remarkable set of historical documents.

Until the famous wartime conference in Tehran in 1943, FDR and Stalin had never met and, unlike Winston Churchill, Stalin had never visited the United States. The two men could not have been more different. FDR was the patrician head of a representative democracy; Stalin, the friend and collaborator of Lenin, was the son of peasants from Georgia and had been exiled to Siberia for his revolutionary activities against the tsar.

There were no illusions in America about “Uncle Joe,” who, after all, had in 1939 entered into a nonaggression pact with Germany. Roosevelt himself had called Stalin’s Soviet Union an “absolute” dictatorship.Yet he apparently believed he could charm the Georgian into a lasting international community after the war.

When the time came, and Germany violated its nonaggression agreement and invaded the Soviet Union in June 1941, FDR put aside his ideological differences and was determined to help Stalin resist Hitler, despite naysayers in his administration that doubted the Soviet Union could last out the year. The country was in dire need of supplies and equipment to withstand the Nazi assault. As Stalin surrendered miles of Soviet territory, he asked the Allies for help.

FDR responded in September 1941 with a Soviet version of Lend-Lease, the program the United States adopted to help the United Kingdom earlier that year. But unlike the agreement with Britain, which provided for the use of British bases in exchange for destroyers and other supplies, this time there were no strings attached, not even the use of Soviet bases.

“I deem it of paramount importance,” FDR wrote in August 1941, “for the safety and security of America that all reasonable munitions help be provided for the Soviet Union, not only immediately but as long as she continues to fight the Axis powers effectively.” Harry Hopkins, one of the president’s closest aides, was dispatched to meet Stalin in July to cement the deal.

The letters, supported by Ms. Butler’s commentary and superb editing, and including a foreword by Arthur Schlesinger Jr., take us through the early stages of the program, as Stalin sets out his needs and Roosevelt combats opposition at home and abroad to relieving the Soviet Union. For the next several years, the flow of supplies to the Soviet Union continued unabated, and included even the loan of British submarines and elements of the Italian navy after the fall of Italy.

The details of the shipments (FDR to Stalin, October 13, 1941: “We are shipping 100 bombers and 100 of our newest fighter planes with spare parts and ammunition. These will be placed on ships during the next ten days”; January 9, 1943: “I have arranged that two hundred C-47 transport planes be assigned to you in January”) illustrate the tremendous scale of the operation.

The logistical problems were enormous and dangerous, especially in late 1941 and 1942, when German U-boat attacks devastated the freighter shipping supplies to the Soviet Union as it slowly turned back the Nazis.There is no doubt that these shipments enabled the Soviet Union to bring its seemingly endless manpower reserves to effective use.

Aside from these details, the correspondence covers a number of other subjects, including a possible meeting between Stalin and FDR. While there was a steady flow of high-level aides and ambassadors to the Soviet Union transmitting FDR’s letters, Roosevelt considered it crucial to the postwar situation that he meet with Stalin. Indeed, Roosevelt requested to meet Stalin without Churchill present to discuss the war and its aftermath.

Churchill, in contrast, had met with FDR several times during the war, in Washington, Quebec, and, most importantly, in Casablanca. At this last meeting, in January 1943, to which Stalin was invited but could not attend, the Allies called for the unconditional surrender of their enemies and promised Stalin that a second front would be opened in Europe that year. That second front did not actually open until the Normandy invasion in June 1944.

Stalin appreciated the invasion of Italy the year before, but did not think it was driving enough German forces away from the East, where millions of Russians had been dying for the past two years, a fact he raises in almost every letter.The gentle parry and thrust of Roosevelt (at times writing with Churchill) in explaining the delays in opening the second front are among the most interesting in the volume.

On the surface, the letters are interesting enough, but there is another story. It becomes clear that what FDR is really concerned about is what happens after the war. With Germany and Japan defeated, and Europe exhausted, only the United States and the Soviet Union remained strong enough to influence the postwar world. Ms. Butler and Mr. Schlesinger focus on what FDR set out to achieve.

There were, they argue, three goals: Soviet membership in the proposed United Nations, the entry of the Soviet Union in the war against Japan, and the declaration of liberated Europe, which supposedly bound the Russians to the right of self-determination for the people of Eastern Europe. Of course, the declaration was essentially a dead letter, and Mr. Schlesinger acknowledges that Stalin violated the postwar accords settled on at Yalta almost immediately. Nor did the creation of the United Nations, and the Soviet Union’s presence in it, do much to bring it within a free international community.

Ms. Butler and Mr. Schlesinger are firmly in an internationalist camp and explicitly place FDR in opposition to the natural reluctance of America to remain engaged in foreign affairs. It was important, therefore, for FDR to achieve his goals as soon after the war as possible, before that nativism of America reasserted itself. The pressure on FDR to do so seems to have distracted him from dealing with the true intentions of his wartime allies.

Ms. Butler takes us through the death of FDR and Churchill’s famous 1945 “Iron Curtain,” showing how opposition to the Soviet Union solidified after the war. She blames Truman for ignoring the Soviet contribution to the war against Japan, in particular its invasion of Manchuria once Germany had been defeated. She also treats Truman as something of a rube (sophisticated diplomats like Hopkins are treated much better).

Such partisanship – one gets the sense, at least from Mr. Schlesinger’s foreword, of a need to settle old scores – mars what is otherwise an important scholarly contribution. But even from the sometimes one-sided account given here, it is clear Truman (and Churchill, and others such as George Kennan) was right about the Soviet Union and its duplicity. FDR, by contrast, was carried away by his belief that he could persuade the Soviets.

Mr. Russello lives in Brooklyn. He last wrote in these pages on the New Journalism.