TOKYO — The U.S. needs troops in Japan for the long term to keep China’s rising power in check and contain North Korea’s aggressive nuclear and missile aspirations, Defense Secretary Robert Gates said Friday, even as the U.S. begins considering a future with a smaller, more affordable military.

“On account of the scope, complexity and lethality of these challenges, I would argue that our alliance is more necessary, more relevant and more important than ever,” Gates said in a keynote speech at Tokyo’s Keio University.

Without U.S. armed forces in Japan, Gates said North Korea’s military provocations could be “even more outrageous,” China might act more aggressively, disaster assistance would take longer, joint exercises would be harder to execute and the U.S. would have less intelligence on the region.

In a sweeping assessment of the U.S.-Japan security stance, Gates said the two countries require “more effective” missile defense capabilities. He praised the existing system as “one of the most advanced of its kind in the world,” and hailed the advanced SM-3 interceptor’s ability to thwart a North Korean attack. Both countries will continue to share missile defense commands at Yokota Air Base.

Gates made clear that the U.S. intends to move forward with Futenma relocation under the plan adopted last year, although he took a softer tone in remarks Thursday and at the defense ministry. That marked a change from past visits in which Gates had pressed Japan to get on with the move.

The address marked the last day of a weeklong swing through Asia where, in Beijing, Gates won Chinese support for better U.S. military relations and cooperation on North Korea. Gates left Japan on Friday for Seoul, South Korea, and brief visits with President Lee Myung-bak and National Defense Minister Kim Kwan-jin.

Gates said the region’s security challenges dominated conversations during his trip, but he defended his decision to engage in more open military relations with China.

“I disagree with those who portray China as an inevitable strategic adversary of the United States,” he said. “We welcome a China that plays a constructive role on the world stage.”

Yet, he warned that Chinese advances in cyber and anti-satellite warfare pose “a potential challenge” to U.S. military’s Pacific operations and communications “in ways that could inflict enormous damage to advanced, networked militaries and societies. Fortunately, the U.S. and Japan maintain a qualitative edge in satellite and computer technology,” he said.

China dominated the questions from the Japanese audience questions. On the perceived gap between China’s civilian and military leadership, given Tuesday’s J-20 stealth fighter test that appeared to surprise China’s President Hu Jintao, Gates called the disconnect worrisome. But he said repeatedly that he believes Hu is clearly in command.

Hu visits Washington next week to meet with President Barack Obama.

Speaking a week after introducing Pentagon plans to reduce the size of the U.S. military and pull back on costly weapons buys, Gates said Japan will need to take on more of its security responsibilities.

“In the United States, we are engaged in a robust debate about the size, composition and cost of our military,” Gates said. “We will continue to maintain the military strength necessary to protect our interests, defend our allies and deter potential adversaries from acts of aggression and intimidation. To do this, we need a committed and capable security partner in Japan.”

Gates praised Japan’s recent agreement to stand “shoulder to shoulder” with South Korea, saying their increased military activities around the world are a key rationale for recommending Japan join the U.N. Security Council.

For now, regional security remains wholly dependent on the U.S. Yet as Gates openly advocated for negotiations with North Korea, he sounded pessimistic.

“Despite the hopes and best efforts of the South Korean government, the U.S. and our allies, and the international community,” he said, “the character and priorities of the North Korean regime have sadly not changed.”

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