Japan will arm up (currently spending only about 1% of their GDP on defense--I think they recently broke that symbolic long-standing limit by a small amount) if China persists in their course. This is only partly true:
What does Japan have now? Her self-defense force numbers at about 250,000. At present, she lacks any but defensive armaments. Even so, her advanced technological capabilities mean that she can develop herself any weapon she needs, as good or better as the American systems on which she now largely relies. Japan does not steal technologies. She already has her technologies.
It is not true that Japan has only "defensive" armament. Shooty things can be used for offense and defense, and is only a legalistic formulation to keep Japan's sort-of-military--if you dim the lights and squint--legal under their pacifist constitution. Case in point is Japan's latest "helicopter destroyer" that looks suspiciously like a small carrier, complete with angled flight deck, and which could carry F-35Bs.
Which this subsequent point in the initially linked article makes for me:
The jewel in her crown is her small (19) submarine force. The Sōryū is a conventional submarine so stealthy that the highly skilled Japanese anti-submarine forces can find only 5% of them when under way. They regularly sink American carrier escorts (using lasers) in war games. More importantly, as retired Chinese General Liu Yazhou 劉亞洲, an adamant Japanophobe, has warned, in case of naval conflict today, the Japanese submarines could sink the entire Chinese East Sea fleet in four or so hours.
That's pretty good for "defensive" weapons.
What Japan really lacks is the ability to sustain combat or engage in it very far from their own home islands without American assistance. Japan will proceed to gain those capabilities given enough time.
And quite possibly nukes because the Japanese won't rely on America risking Los Angeles to strike back at North Korea or China if either threatens to nuke Tokyo.
Russia, which occupies Japanese islands in the north (since World War II), will no doubt be really happy with their so-called ally China for increasing the chance that Japan will resort to force to recover that territory should negotiations fail.
UPDATE: A coup sponsored by China or invasion by China (whether joined by South Korea and Russia, or not) is better than an aerial strike campaign that could potentially lead to a state or regime collapse and resulting massive refugee flows to China. China doesn't want that, either.
So China doesn't want a nuclear-armed North Korea that will prompt nuclear proliferation in potential foes of China; and China doesn't want North Korea to send large numbers of refugees to China in a collapse of state authority.
Which seems to leave a coup that puts a non-nuclear but pro-Chinese state into power in Pyongyang or an invasion that forcibly achieves that--and a joint invasion avoids putting China in the spotlight to see potential weaknesses in their shiny new military as well as sharing costs for rescuing North Korea from the poverty and oppression that has traumatized the nation.
There is more on China at the link about non-Korea issues, too.
