RUTHFULLY YOURS

First, a little more history: The U.S. could reach Japan only after the Marines took the Mariana Islands at great cost in 1944. The largest airports in the world were built within months and filled with new, modern B-29 bombers. The B-29 was a marvel and the greatest expense of the war at $3 billion, compared with $2.4 billion for the Manhattan Project. Each plane was three times the size of the next-largest bomber, the B-17. The B-29 could fly 3,700 miles and cruise at an altitude high enough to elude antiaircraft fire.

But the B-29 ran into a problem during its first mission over Japan—huge winds up to 200 miles an hour. The jet stream rendered it impossible for bombs to hit targets.

Hap Arnold, head of the Army Air Force, turned to his youngest general, 38-year-old Curtis LeMay, who didn’t fit the profile of the glamorous flyboy. LeMay was slightly overweight, surly and taciturn. Most people found him frightening. He was a lieutenant in 1940 but rose out of obscurity to become the most innovative problem solver in bomber command. LeMay also insisted on flying the lead bomber on every dangerous mission. He was also the only U.S. general in the war who fought in front of his troops—a case study in military leadership.

LeMay approached the jet-stream dilemma like the engineer he was. On the night of March 9, 1945, he sent 346 B-29s to Tokyo. In a radical departure from normal operations, he ordered the planes to fly low—5,000 feet—and not in formation, but in a single-file line. The planes would drop incendiaries instead of impact bombs. The crews protested, assuming they would be destroyed by the flak. But LeMay believed the crews could survive because the Japanese wouldn’t see this coming.

He was right. With minimum loss to the U.S., the incendiaries started a firestorm that burned down more than 16 square miles of Tokyo. The firestorm left more than a million homeless and killed an estimated 100,000 men, women and children. The Japanese were as surprised as they were devastated. More people died during that 24-hour period than perished five months later in either Hiroshima or Nagasaki.

If this sounds shocking to contemporary ears, consider the context. An estimated 15 million to 17 million Asian civilians were killed—Chinese, Koreans, Filipinos and those from every other country Japan conquered. Like their German allies, the Japanese adopted a sense of triumphal racial superiority. Many of their victims were killed in the most brutal, medieval ways. An average of 250,000 people were dying throughout Asia every month in the first half of 1945.

As Americans approached the mainland, the Japanese fought even more ferociously on Iwo Jima and later Okinawa. The war in the Pacific was turning into an out-of-control bloodbath. The only way to stop this mass death—and prevent a prolonged guerrilla war following the largest invasion in history—was to force the empire to surrender by destruction from the air.

The U.S. would have to firebomb 64 Japanese cities, capped off by the two atomic bombs in August 1945, to end World War II. In the tragic calculus of war, it took the deaths of untold numbers of human beings to save the lives of even more. These are brutal realities few people today can imagine, let alone confront.

Sept. 2 will mark the 75th anniversary of the Japanese surrender on the USS Missouri. That happened without an invasion of the Japanese mainland. Imagine Iwo Jima times 100 or even 1,000. At least a million American servicemen and many more millions of Japanese lived full lives thanks to the terrible and tragic—but necessary—events that began on March 9, 1945.

Mr. Kozak is author of “LeMay: The Life and Wars of General Curtis LeMay.”