The predictable results of Japan’s snap parliamentary elections give then-and-now Prime Minister Shinzo Abe plenty of space to push constitutional changes that expand the reach of his powerful military. If Abe and the victorious Liberal Democratic Party get the changes they want by revising the Constitution’s Article 9, Japan’s armed forces -- already the world’s seventh strongest -- will be able to strike more proactively.
Japan invaded much of China from 1931 through the end of World War II in 1945, and today’s Chinese leadership believes the invader hasn’t properly atoned for that era.  Even if constitutional reform in Japan retains wording that promises a pacifist spirit for the military, any broadening of its actual power will anger Beijing. That's because China is vying with Japan over a disputed tract of sea as well as crucial influence in other parts of Asia.
The 1947 Constitution following Japan’s World War II surrender commits troops to purely defensive purposes. Article 9 now says “Japanese people forever renounce war as a sovereign right of the nation" and that “land, sea, and air forces, as well as other war potential, will never be maintained.”

But maybe they will be maintained someday. If you look at shifts since 2004. “Japan has been gradually upgrading its defense posture for many years, including strengthening its alliance with the United States, becoming more involved in maritime security efforts, aiding U.N. peacekeeping missions, employing military assets in disaster response (and) dealing firmly with North Korea and displaying a visible ‘hedge’ vis-à-vis China,” says Leif-Eric Easley, associate professor of international studies at Ewha University in Seoul.
Abe says he wants to go ahead with constitutional changes now, per media reports in Japan.
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China is determined to oppose Japan's expansion
China’s official Xinhua News Agency warned after the snap elections, via its analysts, that a change in constitution would eventually let Japan become a military power again despite a post-World War II order imposed by the United States.
“The Chinese…will protest vigorously, labeling PM Abe a militarist, revisionist and an anti-China leader determined to contain China hand in hand with the U.S. to prevent the rightful great rejuvenation of the Chinese nation and people,” says Stephen Nagy, senior associate professor of politics and international studies at International Christian University in Japan.
The Communist government has made World War II history such a big issue that its anti-Japanese pronouncements fanned destructive mass demonstrations around China in 2005. More violent protests erupted in China in 2012.
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China probably knows Japan is unlikely to start a war. More pressing is the facts that since 2013 Beijing has routinely sent military planes and ships to challenge Japan’s control over eight uninhabited islets in the East China Sea. China claims the islets, which it calls the Diaoyu, as well undersea natural gas fields nearby despite the fact that the United States gave the islets to Japan in 1972. More empowered Japanese naval and air force missions around the islets it calls the Senkakus could better resist the same from China.
The widening reach of Japan’s military also threatens China’s overall superpower status in Asia. Like China, Japan is spreading development aid far past its own borders, including arms sales to Southeast Asian countries that dispute Beijing’s maritime claims. Also to that end, Japan sent a helicopter carrier earlier this year to shake Chinese dominance in the contested South China Sea.
Both China and Japan like to be seen as good world citizens by joining UN peacekeeping missions, which also earns them favor with the weaker nations. China has committed one-fifth of the international organization's 40,000 stand-by troops. The foreign ministry in Tokyo says 10,045 Japanese personnel had helped U.N. peacekeeping missions through 2015.
Japan's lead will inevitably be its old military friendship with Washington. Any constitutional change that lets Japanese Self-Defense Forces work more closely with U.S. counterparts would particularly rile Beijing, which still sees Washington as its chief post-Cold War rival in Asia. The U.S. military outranks China’s and whoever it backs naturally gets stronger. Abe wants a clear definition for the Japanese Self-Defense Forces "to allow for the opaqueness of when and how it is dispatched to be much more transparent," Nagy says. That includes the details of cooperation with U.S. forces. "Changing the law will allow Japan to work more seamlessly with its U.S. alliance partner," he says.