In late 2017 Japan's cabinet made the critical decision to purchase two land-based missile facilities designed to shoot down incoming ballistic missiles. The system, known as Aegis Ashore, will protect the country from North Korean ballistic missiles and augment the country's ability to defend its airspace. But the system's usefulness doesn't stop there as Aegis could also protect the country—and American bases—from missiles fired from China.

Japan has been living with the North Korean missile threat for decades. Just 450 miles from North Korea, Japan has been within range of Pyongyang’s missiles since 1994 when North Korea’s Hwasong-9 (otherwise known as the Scud ER) entered service. In addition, Japan was a former colonial occupier of Korea and so holds a special place in North Korea’s revolutionary ideology and history. Added to this is Tokyo’s mutual defense pact with Washington and support for the government in Seoul, which ensure that, despite being avowedly pacifist, Japan remains high on Pyongyang’s enemies list. North Korea has also fired multiple ballistic missiles over Japanese territory, including an August 28, 2017 test of a Hwasong-12 intermediate range ballistic missile that overflew the northern island of Hokkaido.

Japan is the only other country in the world, aside from the United States, to enjoy a tiered ballistic missile defense system. Area defense against ballistic missiles is provided by four Kongo-class guided missile destroyers equipped with the Aegis Combat System and SM-3 missile interceptors. Besides, local, last-ditch defense of cities, military bases, and other vital locations are provided by Patriot PAC-3 short-range interceptors.

Typically, just two Kongo destroyers can protect all four of Japan’s home islands. The increasing complexity of the threat, however, as well as the rise of China’s navy means that the highly capable Kongos would likely be needed elsewhere. Replacing them as Japan’s area ballistic missile defense will be two Aegis Ashore facilities, both on the island of Honshu, one in the north at Akita and another farther south at Yamaguchi. The two facilities should be operational between 2022 and 2023.

In addition to SM-3 interceptors, Japan is also interested in buying SM-6 surface-to-air missiles to provide a defense against cruise missiles. Developed initially as a shipboard weapon, SM-6 is a versatile defensive weapon capable of engaging aircraft, incoming anti-ship missiles, cruise missiles, and even short-range ballistic missiles. (Curiously, North Korea has no cruise missile capability of its own.)

Furthermore, North Korea is not Japan's only potential adversary. After all, China fields a broader spectrum of missiles, including CJ-10 and CJ-20 cruise missiles. Relations between the China and Japan have also soured in recent years after Beijing began pushing claims to the Senkaku (Diaoyu) islands in the East China Sea and virtually the entire South China Sea. In a war with China, or even a war between the United States and China, Chinese cruise and ballistic missiles would almost certainly be used to target vital U.S. and Japanese military facilities across Japan.

This means that Japan's Aegis Ashore facilities could grow much more extensive as they expand to meet various threats. For instance, Aegis Ashore consists of the Aegis Combat System, SPY-1D radar, and Mk. 41 vertical launch system silos, each loaded with one SM-3 interceptor. The number of Mk. 41 silos in each system ranges from twenty-four at the Aegis Ashore facility in Romania to up to 122 on the U.S. Navy’s Ticonderoga-class guided missile cruisers. How many Mk. 41 silos Japan plans to order is unknown, but each Aegis facility could theoretically control hundreds of silos loaded with SM-3 and SM-6 missiles.

Besides foreign threats, geography may also eventually force further expansion of Japan's Aegis Ashore systems. Although the Yamaguchi base's SM-3 interceptors can defend Okinawa and the rest of the Ryukyu Island chain from ballistic missiles, the SM-6 lacks the range to defend that far south. Therefore a third Aegis Ashore base on Okinawa would protect U.S. and Japanese facilities from cruise missile attacks. The base would also safeguard the Ryukyus from tactical aircraft attacks such as from Chinese H-6K bombers which regularly pass through the nearby Miyako Strait just 150 miles from Okinawa.

Meanwhile, another base on the northern island of Hokkaido would grant similar protection. It might further be worthwhile to build a separate base to protect the thirty million residents of the greater Tokyo metropolitan area and yet another to protect Osaka, Kyoto, Nagoya, and other cities in the central Kansai region. Although expensive, each subsequent system would also introduce a level of redundancy into the national network, in case an enemy attack crippled one or more Aegis base.

Aegis Ashore is set to become a cornerstone of Japan's defensive strategy and indeed a shield for the entire nation. Strictly defensive in nature, Aegis is entirely consistent with Japan's pacifist leanings. Japan's new air defenses will go a long way towards avoiding the horrors of another destructive war on Japanese soil.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he co-founded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: Reuters