North Korea announced on Monday that it has tested a nuclear bomb is set to push Japan to expand its own military and stir debate on what was once the ultimate taboo of developing atomic weapons itself.
The test comes as Japan gradually expands its defence posture, 60 years after it was defeated in World War II and forced by the United States to renounce the right to a military.
Prime Minister Shinzo Abe, who took office just two weeks ago, is a sworn hawk on North Korea who has long supported a larger role for Japan's military alongside its ally the United States.
Analysts expect North Korea's test to boost the hand of Abe, who wants to rewrite the pacifist 1947 constitution and allow Japanese troops to engage in overseas operations alongside allies.
Despite its pacifism and US guarantees to protect Japan, the country now has around 240,000 troops on active duty and an annual military budget of 4.81 trillion yen (41.6 billion dollars).
A draft new constitution would preserve Japan's official pacifism but acknowledge it has a military - not the "Self-Defence Forces" as it is now known.
Japan has already been taking a larger international military role. It sent a small but symbolic reconstruction mission to Iraq, the first time since World War II that Tokyo has deployed in a country where fighting is underway.
Japan is also believed to be capable of assembling nuclear weapons if it makes the political decision.
But it would be a drastic change of policy for Japan, the only nation to suffer nuclear attack, which has long campaigned to eliminate atomic weapons.
More than 210,000 people were killed in the 1945 US atomic bombings that flattened Hiroshima and Nagasaki.
"I can't reject the possibility that a nuclear deterrent system would be developed in the region," said Yoshinobu Yamamoto, a professor of international politics at Aoyama Gakuin University.
"Even if the North's missiles do not reach the United States, they could easily put Japan in the firing range and destroy it," he said.
Former Prime Minister Eisaku Sato proposed developing nuclear weapons in the 1960s as China built the bomb. But his position was rejected by the United States, which provides a security umbrella over Japan.
More recently, a magazine in 2006 quoted Foreign Minister Taro Aso as telling US Vice President Dick Cheney that Japan would need atomic weapons if North Korea pursued a nuclear program. Aso's aides denied the report.
Most Japanese support some revision to the constitution. But the country is sharply divided on how far to deviate from official pacifism.
A recent study by a US House of Representatives committee on intelligence said that Japan - and also South Korea and Taiwan - could be driven to pursue nuclear weapons if North Korea tests an atomic bomb.