Here are some facts about the devastating attacks.
The bombs
The first atomic bomb was dropped on the western city of Hiroshima by the US bomber Enola Gay, nicknamed “Little Boy”. It detonated about 600m from the ground, with a force equivalent to 15,000 tonnes of TNT.
Tens of thousands died instantly, while others succumbed to injuries or illness in the weeks, months and years that followed.
Three days later the US dropped a second bomb, dubbed “Fat Man”, on the southern city of Nagasaki.
The attacks remain the only time atomic bombs have been used in wartime.
The blasts
In Hiroshima, the first thing people noticed was an “intense ball of fire”, according to the International Committee of the Red Cross (ICRC).
Temperatures near the blast reached an estimated 7,000°C, which incinerated everything within a radius of about 3km.
“I remember the charred bodies of little children lying around the hypocentre area like black rocks,” Koichi Wada, a witness who was 18 at the time of the Nagasaki attack, said of the bombing.
ICRC experts say there were cases of temporary or permanent blindness due to the intense flash of light, and subsequent related damage such as cataracts.
A whirlwind of heat generated also ignited thousands of fires that ravaged large parts of the mostly wooden city. A firestorm that consumed all available oxygen caused more deaths by suffocation.
It has been estimated that burn- and fire-related casualties accounted for more than half of the immediate deaths in Hiroshima.
The explosion generated an enormous shock wave that blew people through the air. Others were crushed to death inside collapsed buildings or injured or killed by flying debris.
Radiation effects
Radiation sickness was reported in the aftermath by many who survived the initial blasts and firestorms. Acute symptoms included vomiting, headaches, nausea, diarrhoea, haemorrhaging and hair loss, with radiation sickness fatal for many within a few weeks or months.
Survivors, known as “hibakusha”, also experienced longer-term effects including elevated risks of thyroid cancer and leukaemia, and both Hiroshima and Nagasaki have seen elevated cancer rates.
Of 50,000 radiation victims from both cities studied by the Japanese-US Radiation Effects Research Foundation, about 100 died of leukaemia and 850 suffered from radiation-induced cancers.
The group found no evidence however of a “significant increase” in serious birth defects among survivors’ children.
The aftermath
The twin bombings dealt the final blow to imperial Japan, which surrendered on Aug 15, 1945, bringing an end to World War II.
Historians have debated whether the bombings ultimately saved lives by bringing an end to the conflict and averting a ground invasion.
But those calculations meant little to survivors, many of whom battled decades of physical and psychological trauma, as well as the stigma that sometimes came with being a hibakusha.
Despite their suffering, many survivors were shunned – in particular for marriage – because of prejudice over radiation exposure.
Survivors and their supporters have become some of the loudest and most powerful voices opposing nuclear weapons, including meeting world leaders to press their case.
Last year, the Japanese anti-nuclear group Nihon Hidankyo, a grassroots movement of hibakusha, won the Nobel Peace Prize.
In 2019, Pope Francis met several hibakusha in Hiroshima and Nagasaki, decrying the “unspeakable horror” and calling for the abolition of nuclear weapons.
In 2016, Barack Obama became the first sitting US president to visit Hiroshima. He offered no apology for the attack, but embraced survivors and called for a world free of nuclear weapons.
Russia is one of around 100 countries expected to attend this year’s memorial in Nagasaki, the first time Moscow has been invited to commemorations in the city since the start of the war with Ukraine.