The unprecedented visit to Hiroshima by then U.S. President Barack Obama last year was decided by himself several days before an official announcement, with strong support from his senior advisers, especially, then U.S. Ambassador to Tokyo Caroline Kennedy, according to Obama's top aide.

"We had a final discussion in which he made the decision in the Oval Office -- it was our morning meeting with his senior White House national security staff. I think it was early May, a week or two before the announcement," said Ben Rhodes, a speech writer and former Deputy National Security Adviser for Obama, in a recent interview and follow-up email exchanges with Kyodo News.

Obama visited Hiroshima on May 27, 2016, following the Group of Seven summit in central Japan. The official announcement was made by the U.S. and Japanese governments on May 10 that year.

"(In the early days of the administration,) he made clear to us that he was interested in going to Hiroshima, that he wasn't going to 'close the door on' the possibility of going to Hiroshima...President Obama always indicated that he was interested in going," Rhodes added.

An initial internal discussion about a potential presidential visit to the atomic-bombed western Japan city was made in the fall of 2009, when Obama made his first official visit to Tokyo.

Although the option was not seriously considered because the trip to Japan was so short, that was a significant starting-point toward the realization of the historical visit to Hiroshima six and half years later.

"What we were surprised to learn, frankly, is that the United States had not been represented, at any type of high level, at the annual commemoration of Hiroshima," said Rhodes who drafted Obama's 17-minute Hiroshima speech as well as the Nobel Peace Prize-winning "Prague Speech" in April 2009.

In Prague, Obama made a strong commitment to "seek the peace and security of a world without nuclear weapons" and emphasized the need to act from "moral responsibility" as the only nuclear power to have used nuclear weapons ever before.

Hiroshima and Nagasaki, devastated by U.S. atomic bombings during the final phase of World War II in 1945, are the only cities to have had nuclear weapons used on them.

After 2010, Obama continued to send senior officials including Kennedy and her predecessor, John Roos as well as Undersecretary of State Rose Gottemoeller, to Hiroshima and Nagasaki to attend commemoration ceremonies in the two cities on Aug. 6 and 9.

Just a month before Obama's trip, then Secretary of State John Kerry made his own visit to Hiroshima as the first sitting secretary of state, laying important groundwork for Obama's visit.

"It was a sequence...It took us a little longer, frankly, than I thought. The challenge, when you're a U.S. President, is you have so many things that you're doing that it's hard to find time, frankly. But we always, I think, envisioned that he would go, while he was President," Rhodes said.

Obama is well known for his interest in nuclear disarmament and non-proliferation agendas. He launched the Nuclear Security Summit and thought of adopting the "no first use" policy, which means that the sole purpose for possessing nuclear arsenals is to deter other nations from using nuclear weapons.

Rhodes said he thought such a policy was "the direction that (Obama) was going in."

"Based on conversations with our military, they weren't comfortable going that far, and I think he didn't want to be too far ahead of the military. But, I think he certainly wanted to signal a direction towards using them for deterrence," Rhodes added.

He also noted moral factors played a role in Obama's nuclear policy. "He felt like it was important to embrace that moral component, so it wasn't just a security issue; it's a moral issue, given the risk that comes along with nuclear weapons, given what we saw in Hiroshima and Nagasaki."

After Obama left the White House, his successor, President Donald Trump, told Reuters in February that he wants to ensure the U.S. nuclear arsenal is at the "top of the pack," saying the United States has fallen behind in its weapons capacity.

Rhodes expressed concern about Trump administration's stance on nuclear issues.

"My concern is that Trump -- who has been very focused on military spending -- is more likely to pursue much more robust (nuclear) modernization...the combination of Trump and (Russian President Vladimir) Putin means that disarmament between the U.S. and Russia is unlikely in the next 3-1/2 years."

"What Hiroshima shows us is that, given the development of technology, if we don't stay committed to getting rid of nuclear weapons, we're going to end up back here. There will be another Hiroshima," he warned.

"So, my point is that it's easy to lose hope and say 'Trump is President, Putin is President, and so this cause is lost,' but we need to stay -- we need to constantly remind people of what happened in Hiroshima, because that shows the cost of nuclear weapons."