Japan was already badly weakened; the atomic bombs and the Soviet declaration of war in August 1945 pushed its leaders to surrender.
By summer 1945, Japan had suffered military defeats, heavy losses of shipping and heavy bombing of its cities. A blockade means stopping supplies from reaching an enemy. Losing supplies, plus conventional bombing with non-atomic weapons, weakened Japan's factories and its ability to fight.
The USA then dropped atomic bombs on Hiroshima on 6 August and Nagasaki on 9 August. Their huge destructive power and mass loss of life added the threat of even more destruction.
The Soviet Union had fought Germany as an Ally, but had not yet joined the war against Japan. Japanese leaders hoped it might act as a mediator, helping them negotiate better surrender terms with the other Allies. Instead, the USSR declared war on Japan on 8 August and attacked Japanese forces in Manchuria from 9 August. Japan now faced another powerful army and lost its last hope of a deal.
Japan's leaders were split about surrender. Emperor Hirohito stepped in to support accepting it, which broke the deadlock. Surrender was announced on 15 August and formally signed on 2 September 1945.
Earlier defeats explain why Japan was weak. The shocks of August explain why the decision came then. Historians still debate how much the bombs and Soviet entry each mattered, so explain the effect of each, rather than presenting one as the settled answer.
- 6 and 9 August 1945
- Atomic bombs on Hiroshima and Nagasaki.
- 8 August 1945
- USSR declares war on Japan.
- 2 September 1945
- Japan formally signs its surrender.
Worked example: Weakness versus the decision to stop
Japan was weak before August, but its leaders kept fighting.
- Earlier defeats, blockade and bombing explain why Japan was weak.
- The atomic bombs and Soviet entry explain why leaders decided in August.
- The emperor's move broke the split among leaders.
Watch out for this
Japan signed its surrender straight after the Hiroshima bomb.
Surrender was announced on 15 August and signed on 2 September, after Nagasaki and Soviet entry.
Check your understanding
Why did the Soviet declaration of war matter to Japan?
- It meant the USSR would defend Japan from the USA.
- It came before any Japanese defeats.
- It added a new enemy and ended hopes the USSR would help negotiate.