TOKYO - Many Japanese in the aftermath of the Cold War seriously questioned their country's security alliance with the United States. A decade later, those voices are a lot softer, and one nation deserves much of the credit: North Korea. The fears this week that the mercurial communist regime is preparing for its first test of a long-range missile since 1998 have again illustrated one of the premier rationales for Tokyo's enduring partnership with Washington. Military ties between the two are already tight. Japan is firmly under the U.S. nuclear umbrella, and Washington bases some 50,000 troops on Japanese soil and waters. The two are progressively melding their militaries for greater cooperation. On Friday, Japan and Washington agreed to expand their cooperation on a ballistic missile defense shield. The agreement, signed by Foreign Minister Taro Aso and U.S. Ambassador Thomas Schieffer, commits the two countries to jointly produce interceptor missiles, Japan's Foreign Ministry said. The threatening stance by Pyongyang is likely to strengthen the trend, both by legitimizing the heavy U.S. military presence in East Asia and fueling Japan's recent moves to bolster its own defense posture. "This kind of brinkmanship by North Korea is going to drive public opinion to be more supporting of a closer alliance with the United States," said Gerald Curtis, a Japanese politics specialist at Columbia University. It's not the first time North Korea has brought Tokyo and Washington closer together. Growing worries over Pyongyang's efforts to build a nuclear weapon coincided with a deepening of the military ties in the mid-1990s and a widening of Tokyo's responsibilities to help its ally in the region. But it was North Korea's 1998 test-firing of a ballistic missile over northern Japan into the Pacific that illustrated the archipelago's vulnerability — and fixated the public. Tokyo's immediate response was to launch a spy satellite program and firmly commit to building a joint missile-defense shield with Washington. In the years since, Japan has only grown warier of North Korea. At a 2002 summit in Pyongyang, North Korean leader Kim Jong Il admitted that his country's spies had kidnapped at least 13 Japanese citizens in the 1970s and 80s. Pyongyang returned five of the abductees, but Tokyo says it has not substantiated its claims that the remaining eight were dead — strengthening the general Japanese view of the regime as untrustworthy and unpredictable. A missile test now would carry the added significance of the North's claim to have developed nuclear weapons — a horrifying prospect in Japan, which was attacked with atomic bombs twice by the United States in 1945. In that light, some say a fresh test by North Korea could have an even more dramatic effect on Tokyo than in 1998 — and vanquish the pacifism many ordinary Japanese have clung to since the disastrous defeat in World War II. "A launch would trigger calls for stronger national defense ... and could even lead to a discussion to about Japan possessing nuclear arms," said Takehiko Yamamoto, an international politics expert at Waseda University in Tokyo. Even without a launch, the North Korea card will still continue to play a key role in defense calculations in Tokyo. Security concerns are a major impetus behind Japan's drive in recent years to upgrade its defense capability and raise its military profile internationally. Prime Minister Junichiro Koizumi even used the North Korean threat as a way of illustrating the importance of ties with the U.S. He then successfully argued that Tokyo should preserve those ties by dispatching troops to Iraq in 2004. "Even if you don't have North Korea, you still have a leadership here that wants Japan to be more active on the world scene, including having a somewhat higher profile in terms of its military role," Curtis said. Pyongyang is not the only security concern in a rapidly changing part of the world. Japan is increasingly worried about China's expanding military spending, and Foreign Minister Aso recently irked Beijing by calling China a potential military threat to the region. Whether or not a launch goes ahead, American officials say that in this latest crisis they are reaping the benefits of the moves to tighten coordination between two countries since 1998. "As you've seen by the actions of the past few days, the United States and Japan have consulted extensively on this," Schieffer said at a luncheon at his residence Wednesday. "I think you've seen an unprecedented level of cooperation in sharing of intelligence."