Washington will for the first time send an official envoy to Hiroshima to mark today's anniversary of the city's destruction by a US atomic bomb in 1945, in a move hailed by campaigners as a milestone in the struggle to abolish nuclear weapons.
The decision, which experts say is prompted partly by Barack Obama's personal commitment to disarmament as well as growing regional concerns about nuclear proliferation, was welcomed by A-bomb survivors.
But US ambassador to Japan John Roos and UK deputy ambassador David Fitton, who is also attending, will shrug off demands for an apology and questions about the controversial bombing, which killed more than 140,000 people, mostly civilians.
"The attention is on the ceremony itself and on the victims so we don't want to overshadow the event," said a spokesman for the British embassy in Tokyo. "But given the way the international debate is going, we think this is the right move at the right time."
The two US bombs dropped on Hiroshima and Nagasaki on 6 August and 9 August, 1945, killed an estimated 222,000 people, and continue to cause deaths through cancers and other diseases. Today's ceremony marks the moment - 8.15am on 6 August - when the US B-29 bomber Enola Gay dropped its atomic payload over Hiroshima.
Ban Ki-moon will also become the first United Nations' secretary general to attend the annual peace memorial ceremony. Mr Ban said he was "humbled and moved" after meeting a group of A-bomb survivors. "I hope my attendance sends a strong message to the whole world that we must strive and work harder to realize a world free of nuclear weapons."
The visits come amid rising tensions on the Korean Peninsula between Washington's Seoul ally and the nuclear-armed North. Last year, Mohamed ElBaradei, director general of the International Atomic Energy Agency, and other experts concluded that the North had become a "fully fledged nuclear power" after it conducted another nuclear test in May. Pyongyang threatened last week to retaliate with a nuclear response to joint South Korean-US war games in the area.
Japanese campaigners, long fearful of nuclear proliferation, have been energized by Mr Obama's speech last year in which he pledged to abolish nukes. Hiroshima and Nagasaki believe he could be the first sitting US leader to see the devastation wrought by the bombs. Last November, Mr Obama said in Tokyo that he would be "honored to have the opportunity to visit those cities at some point during my presidency."
Most observers believe, however, that the president's growing political difficulties at home, and the lingering controversy wrought by the decision to drop the bombs at the end of the Second World War would make a first-term visit difficult.
The US still has about 8,000 "active or operational" warheads, each on average carrying 20 times the destructive power of Hiroshima. The other established nuclear powers of Russia, China, France and Britain have been joined by Israel, Pakistan and India, while Iran, politicians in South Korea, Taiwan, Saudi Arabia and even Japan, hint they may one day join the club too. In May, Britain revealed for the first time the extent of its nuclear arsenal disclosing a stockpile of 225 warheads in another move widely hailed as a step towards greater transparency.
Survivors have for years requested that an official diplomatic envoy be sent from the US as a symbolic gesture. "What we would like most is an apology and a promise that the US will ban these weapons," said a spokesperson for the Japan Federation of A-and H-Bomb Sufferers Associations. "We don't think that will happen."
Mr Roos is likely to be questioned today about the legitimacy of the bombings. His decision last year to visit Hiroshima after he took up his post was condemned by James Tibbets, son of Brigadier General Paul Tibbets Jr, who dropped the bomb on the city. Mr Tibbets called the visit an "unsaid apology" and an attempt to "rewrite history."
But Hiroshima Mayor Tadatoshi Akiba welcomed the visit. "The attendance of Ambassador Roos will further strengthen world opinion toward the abolition of nuclear weapons and, we strongly hope, deepen the resolve of the government of Washington, as a nuclear power, to destroy such weapons," he said.