Tokyo (CNN) -- New military guidelines from Japan say North Korea's behavior and China's modernization and "lack of transparency" combine to raise concerns in the region and around the world.
The document released on Friday morning in Tokyo lays out a sweeping overhaul of Japan's defense strategy, bringing it out of the Cold War era to one that sees potential threats from both North Korea and China.
The key shift, state the guidelines, is moving from conventional, heavy forces to a flexible, modern force that includes increasing the number of submarines in Japan's military.
The new defense strategy, set out as a 10-year plan, realigns heavy armored military forces from the northern section of Japan that pointed north to Russia, and beefs up defense in Japan's southwest area with mobile units that could rapidly engage from the south.
The obvious and most immediate threat is North Korea. In a visit to Japan last week, the chairman of the U.S. Joint Chiefs of Staff, Admiral Mike Mullen, urged Japan to take a larger regional role, including joining American military exercises with South Korea.
The defense guidelines mark a formal acknowledgement by Tokyo that the threat of war on the Korean peninsula is reality, says Korea watcher and Keio University's Peter Beck.
"Pyongyang's behavior leaves Japan and North Korea's neighbors no choice but to prepare for the contingency on the Korean peninsula. I do not think that North Korea has a death wish. But when you play brinkmanship, there is always a risk of actually going over the brink. I think there is an acknowledgement in Washington and Tokyo that it could happen."
The Japanese defense strategy states the government views the risk of a full-fledged invasion of Japan as a "low possibility," but that various and complicated causes threaten the security of the country.
Chief among Tokyo's concern are sovereignty issues, highlighted in a recent dispute with China over islands in the East China Sea.
Last September, a Chinese fishing vessel collided with two Japan Coast Guard ships. The collision occurred near disputed islands in the East China Sea, islands both claimed by the countries as their rightful territories. In Japan, the islands are called Senkaku; in China, they are referred to as Diaoyu.
In the wake of the collision, Japan arrested the fishing crew. The coast guard released the crew quickly, but detained the captain. During his detention, relations between Beijing and Tokyo dipped, resulting in the cancellation of high-level bilateral meetings and threats of cutting off critical, rare earth exports to Japan.
"Beijing is building up their military and they have been flexing their muscles," says Beck. "The panda is growling, if you will, and I think that is making Japan very uneasy and looking for ways to strengthen the presence in the south so that they can control their support of Senkaku."
Jiang Yu, China's Ministry of Foreign Affairs spokeswoman, said Beijing wants mutual security, trust and improved relations with Tokyo.
"We hope that Japan as a major country in the region will contribute and put forth a more concerted effort to contribute to peace and stability in the region," says Yu.
Japanese Prime Minister Naoto Kan said a new defense policy should not raise concern among its neighbors.
"It's necessary for the Japanese government to establish our defense in this manner. But that does not mean it's a threat to other countries," said Kan.