North Korea is easy to ridicule. Its portly, rhomboid-haired leader looks like an Austin Powers villain. His over-the-top, bombastic threats sound like Soviet propaganda on steroids. Nighttime satellite photography suggests it can't even power a light bulb. No wonder it's been routinely dismissed as not posing a threat for "at least several more years."

Despite the apparent buffoonery, North Korea is a very real and growing threat to the United States and its allies. Pyongyang has likely already equipped its No Dong medium-range ballistic missile with a nuclear warhead. That puts Japan and South Korea at nuclear risk today. Can North Korea hit the U.S. with a nuclear ICBM? The four-star commanders of U.S. Forces Korea, Pacific Command and North American Aerospace Defense Command all think so. And if not today, then certainly in the very near future.

Kim Jong-un is telegraphing that he will conduct more nuclear and long-range missile tests. Mr. Kim seems desperate to convince experts of his capabilities — by having his picture taken next to a nuclear warhead and while watching a test to prove North Korea has mastered ICBM warhead technology.

There is now near-unanimity of views that stronger sanctions must be imposed on North Korea for its serial violations of international agreements, U.N. resolutions and U.S. law. Even experts and pundits who once derided sanctions in favor of diplomatic engagement now grudgingly admit the necessity of imposing punitive measures on Pyongyang.

South Korea, Japan and the U.S. Congress have all responded strongly to North Korea's nuclear and missile tests this year. Standing up against Chinese pressure and economic blackmail, South Korean President Park Geun-hye moved forward on U.S. deployment of the THAAD missile defense system to South Korea. She also finally pulled the plug on the failed inter-Korean economic venture at Kaesong.

Meanwhile, Japanese Prime Minister Shinzo Abe imposed unilateral Japanese sanctions. And the U.S. Congress overwhelmingly and bipartisanly passed the North Korea Policy and Sanctions Enforcement Act to induce President Barack Obama to move beyond his policy of timid incrementalism by more fully enforcing U.S. law.

The collective U.S., South Korea, Japanese and U.N. punitive measures are welcome, if long overdue, to punish North Korea for its defiance of laws and resolutions. Hopefully they will eventually alter North Korean behavior, but in the meantime, they enforce laws and will constrain both the import and export of prohibited nuclear and missile materials.

With everyone adopting stronger sanctions, Kim Jong-un may perceive himself as being painted into a corner. As a result, he may feel compelled to take even more provocative and desperate steps. With no apparent off-ramp on the highway to a crisis, the danger of a military clash on the Korean Peninsula is again rising.

To face a common threat, the United States must work closely with critical allies South Korea and Japan. Trilateral diplomatic and security coordination is crucial. But while Washington's relations with Seoul and Tokyo are perhaps the best they've ever been, bilateral relations between Japan and South Korea remain strained.

Last December South Korea and Japan reached an agreement to resolve the tragic "comfort women" issue. The accord was achieved through diplomatic perseverance as well as the courage by President Park and Prime Minister Abe. Fulfilling the agreement, however, will require both leaders to push back against nationalist elements in their countries.

The United States welcomed the breakthrough agreement, since it could enable its key Asian allies to refocus attention away from past differences and toward current security challenges. Washington's hope is that progress on this difficult historic issue can lead to expanded military cooperation among the U.S., Japan and South Korea for deterring the growing North Korean nuclear threat.

Last September the National Diet passed defense reform legislation that empowers Japan to play a more comprehensive role in responding to global security challenges. These changes, long overdue and promised by successive Japanese administrations, will allow Japan to augment allied deterrence and defense capabilities. Unfortunately, the changes were perceived by South Korea as dangerous and indicative of an innate Japanese desire to resume a 1930s-style militaristic imperialism.

The Japanese defense reforms pose no threat to South Korea. Indeed, they are critical to implementing the allied defense of South Korea. Japan would provide a critical base of support for U.S. forces involved in any conflict with Pyongyang, and Japanese combat support capabilities would be required during a crisis on the Korean Peninsula. Quite simply, without Japan, the U.S. and South Korea cannot deter a North Korean attack nor successfully defend the Korean Peninsula from northern aggression.

The U.S. has critical national interests in Asia. It needs to remain fully and energetically engaged in the region. But Washington cannot protect these interests on its own. It must rely on its indispensable allies — Japan and South Korea — to achieve mutually beneficial goals. Washington should encourage even greater trilateral security cooperation, including integrated ballistic missile defense systems.