Some 65 years ago, as World War II raged in Europe and the Pacific, the American people faced an unprecedented constitutional crisis of which they were completely unaware -- and which has remained a secret ever since.

It has long been known that President Franklin D. Roosevelt, during the last year of his life, was gravely ill with serious cardiac problems: He'd been diagnosed with acute heart failure in March 1944 and suffered from astronomically high blood pressure and arteriosclerosis.

But what the public did not know was that four years earlier, while still in the second of his four terms as president, FDR had been diagnosed with a deadly skin cancer, melanoma, in a lesion over his left eyebrow.

This disease would metastasize to Roosevelt's abdomen and his brain, causing a tumor that eventually killed him on April 12, 1945.

Which means the cerebral hemorrhage that struck him down shortly before V-E Day was not "a bolt out of the blue," as his doctors contended -- and as historians have long believed -- but the inevitable result of a catastrophic illness, compounded by heart problems.

Dr. Steven Lomazow, a veteran neurologist, and I reached this conclusion -- and others about Roosevelt's health -- after a five-year investigation, the findings of which are in our new book, "FDR's Deadly Secret." How can we be certain? After all, Roosevelt's doctors always denied he had cancer, no autopsy was performed and, save for a few lab slips, FDR's medical file disappeared after his death.

But a careful inspection of hundreds of photos of the lesion indicates a melanoma, according to the late Dr. Bernard Ackerman, the world's foremost dermatopathologist, who worked with us.

Moreover, evidence from Roosevelt’s shockingly inept delivery of his final public speech strongly suggests that he suffered from hemianopia -- the inability to see the text in the left side of his field of vision.

Our book paints a portrait of a president who knew full well that he faced a likely death sentence, yet ignored the odds to run for a third and then fourth term in the White House. Why? Because he believed he could stay alive long enough to see America into and through the coming global conflict, and then to establish the UN.

In fact, in the summer of 1944, Roosevelt was informed in no uncertain terms by Dr. Frank Lahey, one of America's most eminent surgeons, that he would not survive a fourth term. Just 24 hours later, he told Democratic leaders he would run for re-election.

In a sense, FDR rolled the dice with history -- and won.

All the while, despite growing rumors to the contrary, his doctors lied to the American people, insisting that the president was in fine health.

Yet, as we found, there were other serious challenges to Roosevelt's health. In the spring of 1941, for example -- at the time of the Atlantic Charter and of Japan's pre-Pearl Harbor expansion in the Pacific -- FDR spent two months recovering from a life-threatening profound anemia which required as many as nine emergency blood transfusions.

Historians have known of this profound blood loss but never understood its significance -- or realized that FDR had undergone transfusions, which we uncovered in letters between his wife and daughter, as well as evidence on the lab slips. America came within a pint of blood of having Vice President Henry A. Wallace -- who would later run for president in a campaign controlled by the US Communist Party -- in the White House.

And for over a year prior to his death, FDR suffered repeated and dramatic seizures that made him appear almost catatonic and left the many eyewitnesses to such events believing he'd suffered a stroke.

Just how frequently this happened can be seen in the response by Roosevelt’s top aide, Gen. Edwin "Pa" Watson, to a frightened senator who'd witnessed one such attack: "He'll snap out of it -- he always does." Why does all this matter?

Because it raises new questions, long debated by historians, as to whether Roosevelt was fully capable of functioning as chief executive and commander-in-chief during World War II.

Some will see confirmation that FDR was indeed "the sick man of Yalta," incapable of negotiating skillfully with the determined Soviet dictator, Joseph Stalin, on the future of postwar Europe -- a failure, critics have charged, that resulted in a generation of iron-curtain rule over half the continent.

Others will see a determined champion, tirelessly and selflessly guiding America through the worldwide conflagration of WWII while contending with the ravages of the diseases that were taking his own life.

In fact, even as soldiers were fighting the enemy on the battlefields, a parallel struggle was under way in the White House to preserve the president against he considerable medical challenges that surrounded him.

Roosevelt was at the center of this battle -- not as a disengaged and uninterested spectator, as historians have long believed, but as a chief executive who took an active and decisive role in determining the course of his own medical struggle, just as he did in the fight against the Axis powers.

That his risky gamble succeeded for as long as it did hardly excuses the overwhelming danger of the choice he made.