South Korean intelligence believes the North has mastered the technology to place a nuclear weapon on the Nodong medium-range ballistic missile, a weapon that can reach Japan. Although Japan has a two-layered ballistic-missile defense consisting of the Aegis combat system and Patriot missiles, it may find it necessary to add a third, controversial layer: eliminating nuclear-tipped missiles at the source. Here are five weapons systems, both defensive and offensive, that would help Japan deal with the North Korean missile threat. 1. Aegis Ashore: The land-based version of the Aegis combat system that powers both U.S. and Japanese ships, Aegis Ashore would be useful in preventing North Korean missile warheads from falling on Japan. While Japan already has the Aegis ballistic missile defense system on its Kongo-class guided-missile destroyers, there are only four of them. Permanent Aegis Ashore bases would be available 24/7, freeing up the Kongo destroyers for traditional surface-warfare roles. Such a system would also be much cheaper than purchasing new destroyers.