Seventy-seven years ago, on Aug. 9, 1945, a B-29 named Bockscar took off from Tinian, a U.S. air base in the Pacific, to drop a second atomic bomb on a Japanese city. That bombing, using “Fat Man,” a plutonium weapon equivalent to about 21,000 tons of TNT, followed three days after the initial atomic attack, on Aug. 6, of Hiroshima.

The second bomb killed about 40,000-50,000 and probably injured as many in 1945, with more casualties, mostly from radiation, in later years.

That Aug. 9 mission has been often forgotten, and is understudied. What U.S. officials generally concealed in the immediate aftermath is how much went awry on that mission, how it almost failed, and that bad decisions were made.

Indeed, some experts — including Col. Paul Tibbets, commander of the entire A-bombing group (the 509th Composite), much later said that the mission, as problems emerged, should have aborted and returned to base without dropping the bomb.

Ironically, the second atomic bomb was probably unnecessary. It was dropped amid the period of the U.S.’ pummeling conventional mass bombing of Japanese cities, a near-strangling blockade of Japanese shipping and an increasing food crisis in Japan, and right after the devastating first atomic bombing on the 6th, and Soviet entry into the war on Aug. 8.

But no one in the policy-making ranks in Washington, or on Tinian, understood the implications of all that when the second atomic bombing was planned. The dominant assumption was that the second A-bomb, and most likely more atomic bombings, would be required before Japan surrendered.

The desire to quickly drop the second bomb, and thereby to try to speed the war’s end, meant that reasonable standards in planning and in executing the mission were dangerously loosened. That contributed to many of the errors, and many of them were long concealed by the U.S. government.

Even before Bockscar’s takeoff on Aug. 9, a fuel tank problem that morning was discovered, which meant that about an extra 650 gallons would be carried, adding to the plane’s weight, but that gas would not be available for use. Only years later, in a memoir, was that problem revealed.

Despite the bad weather, the plans called for Bockscar to rendezvous en route to the target with two other bombers — one carrying measurement equipment, and the other cameras. No one planning the mission recognized that such a rendezvous could easily go awry in bad weather, and it did. Bockscar wasted about 40-50 minutes, and much fuel, for the failed rendezvous with one of the two planes, named Big Stink.

At some point, a message from Big Stink back to Tinian suggested that the bombing mission had been aborted. That led to the dangerous decision, on Tinian, to remove the air-and-sea rescue craft that had been arrayed for Bockcar if the plane had to ditch.

During the Aug. 9 flight, a monitoring box for the A-bomb suddenly flashed a white light, indicating — to the distress, and probably alarm, of the weaponeer and others — that the bomb was dangerously near imminent detonation. Fortunately, the assistant weaponeer soon figured out that the box’s circuitry — not the bomb’s — was faulty, and that there was no real danger.

To add to problems, the primary target, Kokura — a large arsenal and a city of about 150,000 — was clouded over, thereby barring the required visual bombing. But Bockscar, unwisely, nevertheless spent about 50 minutes making three unsuccessful runs, and consuming more scarce fuel, before heading to the secondary target, Nagasaki.

With only enough gas for one bombing run there, and finding Nagasaki also clouded over, the weaponeer on Bockscar, reportedly after some agony, decided — in likely violation of the rules — to authorize a radar drop, if necessary, because that seemed wiser than ditching the bomb or trying to carry it back to a U.S. base.

But, allegedly, in Bockscar’s bombing run, during about the last 20 seconds or so, the bombardier found a hole in the cloud cover, and he was able to drop “Fat Man” visually. It hit in Nagasaki’s Urakami Valley, and destroyed, among other buildings, much of two large Mitsubishi factories and many smaller plants, and reportedly killed or injured about 45 allied POWs in a camp probably not then known to U.S. intelligence.

The claim that there was actually a sudden hole in the cloud cover would, years later, become a serious question. Adding to the warranted doubts about that claim was the evidence that the bomb probably missed its aiming-point target by about 1.5 miles, or slightly more. That margin of error strongly suggested a radar drop, not a visual drop.

Rather “miraculously,” however, official bombing-target orders, dated Aug. 8, 1945, would later become available that stated that the Urakami Valley plants were, in fact, the target.

Yet, both Bockcar’s weaponeer, Frederick Ashworth, who ultimately became a vice admiral, and the bomber’s pilot, Charles Sweeney, who ultimately became a major general, concluded that those later-discovered orders were “fake.”

There is much to the little-studied story of the Aug. 9 mission that is greatly troubling: the poor decision-making, the long-run concealment of much that went awry, the likely minimization of POW casualties, the important question of the targeting orders, and the larger issue of whether the Nagasaki bombing was unnecessary in producing Japan’s surrender.

Barton J. Bernstein is a professor emeritus of history at Stanford University.