WASHINGTON -- The United States remains committed to the defense of Japan and stands ready to enhance its alliance for regional peace, prosperity and freedom, Defense Secretary Jim Mattis said Saturday in Tokyo.In a joint press conference with Japan's Minister of Defense Tomomi Inada, the secretary said much has changed in Japan since he was stationed there as a young Marine Corps lieutenant."But one thing is certain: The alliance between the United States and Japan is enduring and will remain as the cornerstone of peace and security in the Asia-Pacific region," he said.JOINT REGIONAL CHALLENGES"As the Japanese people know so well, jointly we face many security challenges in this region; from the threat of nuclear missile provocations by North Korea to increasingly confrontational behavior by China in the East and the South China Sea, [and] we recognize the changing security situation," Mattis said.In their earlier joint meeting, the two defense leaders confirmed their intention to continue close coordination on those security issues and others, he said.The secretary said he also expressed to Inada the United States' appreciation for Japan's stabilizing and strengthening efforts with its Southeast Asian partners, which contribute to regional peace, prosperity and freedom.ALLIANCE CRITICAL TO SECURITY"The U.S.-Japan alliance is critical to ensuring that this region remains safe and secure, not just now, but for years to come," he said.The United States' 2015 defense guidelines and Japan's peace and security legislation "lay the foundation for us to do much more together to increase interoperability between our forces and to bolster Japan's capabilities from peacetime to contingency, if needed," Mattis said.The secretary said the coming years will see important strides on both sides toward realizing their mutual goal of a strong defense of Japan and a stable regional environment in which all nations, based on broadly accepted international rules, can prosper, free from fear.The United States has invested in the alliance by deploying its most advanced capabilities to Japan and by maintaining a robust force structure, Mattis said.COMMITMENT TO ALIGNMENT"The United States also remains committed to mutually agreed-upon realignment plans. These include relocating Marines to Guam and reducing our footprint on Okinawa while maintaining the capabilities needed to keep Japan and the region secure," Mattis said.Mattis and Inada also agreed their mutual efforts to build the Futenma replacement facility will continue, with Mattis calling it "the only solution that will enable the United States to return the current Marine Corps air station on Futenma to Japan."Japan has made many noteworthy contributions to regional security and to the alliance, and the United States "deeply appreciates" them, he noted."But make no mistake: In my meeting with Japanese leaders, both of our nations recognize that we must not be found complacent in the face of the emerging challenges," the secretary said."As our alliance grows," Mattis said, "it will be important for both of our nations to continue investing in our defense personnel and capabilities. In this manner, we will ensure that we are true partners today and in the years to come."