World War II transformed the world, but nowhere more than Japan.
The only country in the world to have suffered the devastation of nuclear bombs, Japan rejected the militarism that had defined its politics for decades. In 1947, it adopted a pacifist Constitution, drafted largely by Americans, which formally renounced war as the right of a sovereign nation and the threat or use of force as a means of resolving disputes. Needless to say, Japan would not be seeking to become a nuclear power.
But while the world changed profoundly in the ensuing decades, Japan’s security policy largely did not. The nation’s security remained the responsibility of the United States, which had pledged to defend the country in the event of armed attack. Accordingly, the U.S. maintained a significant troop presence and several bases in Japan, and extended its nuclear umbrella to the country.