TOKYO (AP) — Japan, the only country to have suffered atomic bomb attacks, has repeatedly called for a global ban on nuclear weapons. Yet it sided with the nuclear powers and NATO in refusing to sign a treaty to ban such weapons during the U.N. General Assembly meeting in New York.

The treaty, the first of its kind, was signed Wednesday by 50 countries and would take effect if the same number of countries ratify it. So far three have done so, Guyana, Thailand and the Vatican. Once it takes effect, the signatories would be barred from developing, testing, producing, acquiring, possessing or stockpiling nuclear weapons.

Here's a look at what's behind Japan's reasoning:

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TOKYO UNDER U.S. NUCLEAR UMBRELLA

Japan, as a close U.S. ally, is protected by America's extended nuclear deterrence, or "nuclear umbrella," even though Tokyo renounces its own possession, production or entry of nuclear weapons on its turf. That makes it difficult for Tokyo to sign the treaty especially as it steps up its military role amid North Korea's missile and nuclear threats. Under Prime Minister Shinzo Abe's hawkish government, the two countries have stepped up bilateral security cooperation. Besides Japan, most U.S. allies — almost all NATO members, South Korea and Australia — also did not participate in the talks on the treaty.

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A TREATY LONG PURSUED BY HIBAKUSHA

Most atomic bombing survivors, or "hibakusha," have made lifetime devotion to achieving a nuclear-free world, and their decades-long steady effort has been regarded as a driving force for that cause. Hiroshima atomic bomb survivor Toshiki Fujimori, a leader of Japan's main group for the survivors, says nuclear weapons are absolutely "incompatible" with humans, and the treaty is a first step. He says Japan should sign, and its refusal to do so broke the hearts of many survivors, including his own. During the Aug. 9 memorial service in Nagasaki, the city's outspoken Mayor Tomihisa Taue criticized Abe's government for not joining the treaty, accusing it of making empty promises about achieving a nuclear-free world. He said Japan's absence is "incomprehensible" especially to those in Hiroshima and Nagasaki, where the two U.S. attacks in August 1945 killed more than 210,000 people by the end of that year.

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JAPAN SAYS TREATY SPLITS TWO SIDES, WANTS TO BRIDGE THE GAP

Japan's top government officials say the country did not sign because its approach to achieving a nuclear weapons ban is different from the treaty, even though nuclear abolition is their ultimate shared goal. Foreign Minister Taro Kono told reporters in New York that nuclear weapons states and non-nuclear nations have been divided, and there is even a gap among non-nuclear countries over how realistically they should achieve the goal. Kono said Japan seeks to reach out to both sides in hopes of serving as a bridge and create a common ground where everyone can join for the shared goal of nuclear disarmament and abolition.

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