Unlike Iran, North Korea already has the bomb. It has conducted four underground nuclear test explosions since 2006. The most recent test was in January, when it claimed to have detonated a hydrogen bomb — a much more powerful device than in the previous tests — although the U.S. government doubted that claim.
North Korea is also working on ways to deliver nuclear weapons. After five failures, it successfully test-launched in June a ballistic missile that puts U.S. military bases in South Korea, Japan and Guam within reach. North Korea has displayed an intercontinental missile that could potentially hit the mainland U.S., although it has not yet been flight-tested. It could take several more years to perfect that missile, which can be moved by road, making it harder to destroy pre-emptively. The U.S. military has said North Korea may by now have developed a nuclear warhead small enough to mount on such a missile.
The U.S. keeps 28,500 troops based in neighboring South Korea as a deterrent force, a legacy of the 1950-53 Korean War that ended without a formal peace treaty. North Korea is unlikely to launch a nuclear attack on the U.S. or its allies, a move that would invite its own destruction. But, mindful of the fate of ousted dictators in Iraq and Libya who gave up nuclear weapons programs, Kim is clinging to his. He views atomic weapons as a security guarantee for his oppressive regime. That is the main obstacle to resuming negotiations in which the North could win much-needed aid in exchange for disarming.
International attitudes to North Korea are hardening. The nuclear test in January triggered the toughest sanctions yet, restricting the North's access to foreign currency and weapons technology, but it remains to be seen how aggressively China enforces them. In a sign of how seriously the U.S. takes the emerging North Korean threat, it is investing in missile interceptors in Alaska and California to combat it. U.S. experts estimate that North Korea has 13 to 21 nuclear weapons, and could have as many as 100 by 2020. That's about 20 weapons fewer than what India is estimated to have in its arsenal today.