Rupert Cornwell: The current nuclear threat to Japan is from Pyongyang

Given president Obama's vision of a world one day rid of nuclear weapons, the visit by a US ambassador to Hiroshima to attend today's annual anniversary ceremony marking the use of the atomic bomb against the city in 1945 makes perfect sense. But it is rekindling a debate that rages as strongly as ever in America, 65 years on. Was the use of the bomb justified?

Opponents maintain that the deed was immoral. Some describe it as "state terrorism," tantamount to a war crime. Under no circumstances should the US have employed such a weapon against a country which did not possess one. Nor, they add, was it militarily necessary.

Those who believe Hiroshima and Nagasaki were justified have always insisted that otherwise, the US would have had to invade the Japanese mainland – an operation that might have claimed hundreds of thousands of US lives. But opponents of the bombings retort that captured Japanese documents show Tokyo was on the point of surrender anyway.

Even at the time, such doubts abounded at the highest echelons of the US military. General Douglas MacArthur – to become governor of Japan after the war – believed the bombing was unneccessary. So did General Dwight Eisenhower.

In his memoirs, the future President Eisenhower wrote that when he was told in summer 1945 of Washington's intention to use a nuclear weapon, he argued that such a step was wrong – "firstly because it was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives."

But President Harry Truman had no compunctions, and even now many Americans agree with him. After all, did Japan not start the war with the US with its attack on Pearl Harbor? Having sown the wind, it could have no complaints about reaping the whirlwind.

In 1995 the argument exploded again when the Smithsonian Institution staged an exhibit featuring the fuselage of the Enola Gay, the B-29 that dropped the bomb, to mark the 50th anniversary of Hiroshima. Many American veterans complained the accompanying text put too much emphasis on Japanese civilian deaths. In the end the director of the Air and Space Museum in Washington was forced to resign.

Such arguments may seem remote and academic, given the actual and nearby nuclear threat currently posed to Japan, not by the US but East Asia's rogue state, North Korea. But the presence today of John Roos, the US envoy to Japan, has stirred them anew.