That single bomb killed an estimated 140,000 people and destroyed the Japanese city of Hiroshima.
Witnessing the destruction, Robert Lewis, the co-pilot of the U.S. bomber "Enola Gay," later wrote, "My God what have we done?"
That horror, encapsulated by those few words, continues to resonate worldwide.
Hiroshima and the subsequent bombing of Nagasaki on August 9, 1945, remain the only time atomic weapons of mass destruction have been used in conflict. It effectively brought World War II to an end.
Lessons learned hold for 70 years
Odd Arne Westad, Professor of U.S.-Asia Relations at Harvard University, said he believed the massive destruction, contamination and humanitarian suffering from the attacks on Hiroshima and Nagasaki had an even broader impact by successfully deterring warring nations from using them again for nearly three quarters of a century.
"The world became aware of the terrible consequences and that was very significant. It is remarkable that in the 70 years since Hiroshima, nuclear weapons have never been used again," he told CNN.
"Most people from 1945 onwards would have expected nuclear weapons to be used in warfare reasonably soon after that, yet almost 60 years of Cold War passed without the use of nuclear weapons. If it hadn't been for the lessons both the East and the West learned from Hiroshima, I don't think that would have happened."
Since 1945, when Japan surrendered to U.S. forces, the nation has essentially based its economy and military on a pacifist constitution.
Recent moves by Japan's Prime Minister Shinzo Abe and his ruling Liberal Democrat Party to change the constitution, specifically Article 9 -- the so-called "Peace clause" have met with civil outrage.
Jeff Kingston, Director of Asian Studies, Temple University Japan and author of "Nationalism in Asia Since 1945," said the collective memory of Hiroshima and Nagasaki is an important reason why this powerful anti-war sentiment still exists.
"The Hiroshima Peace Museum is the most visited site for school excursions and every year at this time the media is filled with stories about the nightmarish suffering of the hibakusha (atomic bomb survivors) who eloquently recount what they endured," he told CNN.
He said these pacifist sentiments and distrust of any tampering with Article 9 cross age boundaries.
"'Barefoot Gen' is an iconic manga series targeting youth that focuses on the horrific consequences of Hiroshima and warns against militarism. In short, 'never again' resonates powerfully with the Japanese.
"Article 9 has become a touchstone of Japanese national identity, which explains why Prime Minister Abe has cratered in recent polls because his legislation on Collective Self-Defense is seen as a stealth revision of the pacifist Constitution. Only 18% of the public supports CSD," Kingston added.
Any successful move by Japan to change its long held pacifist stance is likely to make its neighbors in the region nervous, particularly at a time when China is actively reclaiming disputed territory in the South China Sea, souring its relations with Taiwan, Vietnam, Brunei, Malaysia and the Philippines.
In June, China accused the Philippines of roping in Japan over its contentious claims to the Spratly Islands, also known to the Chinese as Nansa. Japan too remains cautious about China's growing presence in the region, wary of the world's second biggest economy gaining a stranglehold over vital shipping routes.
Kingston said it's ironic that public opposition to Abe's CSD indicates they are more worried about him than the Chinese threat.
"Essentially the public worries that somehow, somewhere Japan will get dragged into some war at Washington's behest, so they distrust Abe's evasive explanations. Problematically, in the U.S., Abe assures policymakers that CSD means Japan will be able to do much more in support of the alliance, while in Japan, he tries to convince people that CSD will have no real consequences," said Kingston.
A pacifist nation with nuclear power
Japan currently imports 84% of its energy requirements.
Before the Fukushima nuclear disaster on March 11, 2011, one-third of its electricity was generated from atomic energy. But after the catastrophe -- caused by an earthquake and tsunami -- all reactors in Japan were taken off line.
That holding pattern could change in the near future. Last month, a World Nuclear Association report
stated that 43 Japanese reactors were operable, and another 24 were in the process of seeking government approval.
Brad Williams from Hong Kong's City University's Department of Asian and International Studies, said this highlights the dichotomy of lessons learned from August 6, 1945.
"You could argue that the lessons learned from Hiroshima were somewhat mixed. Japan cannot arm itself with nuclear weapons, but needs to use nuclear energy and rely on U.S. nuclear security guarantees.
"We still see widespread public opposition to any notion of Japan developing nuclear weapons capacity and the nation's reliance on nuclear energy, given the dangers laid bare by the tragic events of 3/11."
Weapons of mass destruction
Although Hiroshima and Nagasaki are credited for steering the world away from nuclear war, since 1945 U.S., Russia, UK, France, China, India, Pakistan and North Korea have collectively conducted around 2,000 nuclear weapons tests.
The Federation of American Scientists estimates there are now more than 17,000 nuclear warheads -- nearly a quarter of those are classed as "operational."
In spite of attempts to prevent nuclear stockpiles, Harvard's Westad claimed there's an increasing proliferation of nuclear weapons, especially in those parts of the world with long term conflict, a situation Westad believes opens up the possibility of them being used again.
"The trouble with having weapons of mass destruction is that if you have got them, it will be tempting, under certain circumstances, to use them," he said.
He pointed to nations experiencing long-term conflict such as North Korea, Pakistan, India and Israel as nations that fit this category.