Introduction: A New Challenge, A New Debate

In January 2011, US Secretary of Defense Robert Gates warned that North Korea's "continuing development of nuclear weapons and their development of intercontinental ballistic missiles (ICBMs) is becoming a direct threat to the United States." He elaborated that the North Korean ICBM threat to the United States is not an "immediate threat" but a "five-year threat."1 His successors have echoed it, with Leon Panetta describing North Korea as a "serious threat,"2 Chuck Hagel saying it was a "real and clear danger,"3 and Ashton Carter emphasizing "how dangerous things are on the Korean peninsula."4 The year 2016 marks five years since Gates issued his warning. Indeed, North Korea conducted its fourth and fifth nuclear tests in January and September 2016, and missile tests - now also from submarines - have continued unabated. The North Korean leader, Kim Jong- un, after the success of an intermediate- range ballistic missile test firing, declared on June 23 that "we have the sure capability to attack in an overall and practical way the Americans in the Pacific operation theatre."5 The US base in Guam is, according to Pyongyang's rhetoric, within the range of a North Korean missile attack. Any nuclear- tipped missiles - whether intermediate- range or intercontinental - would be a grave threat to the United States and its allies in East Asia and raise profound challenges to international security. Yet targeting the United States with a nuclear payload is precisely where North Korea's nuclear ambitions ultimately lie. Pyongyang's five nuclear tests, numerous missile tests, and stated ambition to miniaturize a warhead to fit on a missile and survive the ballistic trajectory, as well as its persistence with developing the synthetic materials required for the warhead to survive reentry, have caused a growing number of analysts to concur with former defense secretary Gates's dire prediction that North Korea can or soon will be able to target the United States with nuclear weapons.6 Indeed, Kim Jong- un revised North Korea's constitution to formally enshrine his country's nuclear status within it and, in his 2015