Barack Obama became the first incumbent U.S. president to visit Hiroshima on Friday, laying a wreath at the site of the world's first atomic bombing in a gesture Tokyo and Washington hope will showcase their alliance and invigorate efforts to end nuclear arms.

Even before it occurred, though, the visit stirred debate, with critics accusing both sides of having selective memories and pointing to paradoxes in policies relying on nuclear deterrence while calling for an end to atomic arms.

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The two governments hope Obama's tour of Hiroshima, where an atomic bomb killed thousands instantly on Aug. 6, 1945, and some 140,000 by the year's end, underscores a new level of reconciliation and tighter ties between the former enemies.

"We come to ponder the terrible force unleashed in a not-so-distant past," Obama said after laying a wreathe at a peace memorial. "We come to mourn the dead."

Before laying the wreath at a peace memorial, Obama visited a museum where haunting displays include photographs of badly burned victims, the tattered and stained clothes they wore and statues depicting them with flesh melting from their limbs.

Aides had said Obama's main goal in Hiroshima was to showcase his nuclear disarmament agenda, for which he won the 2009 Nobel Peace Prize.

"We remember all the innocents killed in the arc of that terrible war," a solemn Obama said.

"We have a shared responsibility to look directly in the eye of history. We must ask what we must do differently to curb such suffering again."

Obama said earlier he would honor all who died in World War II but would not apologize for the bombing. The city of Nagasaki was hit by a second nuclear bomb on Aug. 9, 1945, and Japan surrendered six days later.

A majority of Americans see the bombings as having been necessary to end the war and save lives, although some historians question that view. Most Japanese believe they were unjustified.

The White House had debated whether the time was right for Obama to break a decades-old taboo on presidential visits to Hiroshima, especially in an election year.

But Obama's aides defused most negative reaction from military veterans' groups by insisting he would not second-guess the decision to drop the bombs.

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"I will not revisit the decision to use atomic weapons in Hiroshima and Nagasaki, but I will point out that Prime Minister [Shinzo] Abe and I coming to Hiroshima together shows the world the possibility of reconciliation – that even former adversaries can become the strongest of allies," Obama said in written responses to questions published in the Asahi newspaper on Friday.

Atomic bomb survivors have said an apology from Obama would be welcome but for many, the priority is ridding the world of nuclear arms, a goal that seems as elusive as ever.

Not all agree. "I want Obama to say 'I'm sorry.' If he does, maybe my suffering will ease," said Eiji Hattori, 73, a toddler at the time of the bombing who now has three types of cancer.

"If Obama apologized, I could die and meet my parents in heaven in peace," he said at the peace park, from which ordinary citizens were later ejected amid tight security ahead of Obama's arrival.

Earlier, protesters outside the peace park could be heard demanding an apology.

Five atomic bombing survivors and their families attended the ceremony.

World War II flying ace Dean “Diz” Laird, 95, who shot down Japanese fighters and dropped bombs on Tokyo, said he was pleased Obama was making the visit but glad he wasn't apologizing.

"It’s bad that so many people got killed in Hiroshima, but it was a necessity to end the war sooner,” he said.

Critics argue that by not apologizing, Obama will allow Japan to stick to the narrative that paints it as a victim.

Abe's government has affirmed past official apologies over the war but said future generations should not be burdened by the sins of their forebears.

China and South Korea, which suffered from Japan's wartime aggression, often complain it has not atoned sufficiently.

"It is worth focusing on Hiroshima, but it’s even more important that we should not forget Nanjing," Chinese Foreign Minister Wang Yi told reporters, according to the ministry's website.

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China says Japanese troops in 1937 killed 300,000 people in its then-capital of Nanjing. A postwar Allied tribunal put the death toll at 142,000, but some conservative Japanese politicians and scholars deny a massacre took place at all.

"The victims deserve sympathy, but the perpetrators can never escape their responsibility," Wang said.