Peter brings up an interesting question and one that we don’t yet have a final answer on: Under what circumstances will states balance against another? If shifts in the balance of power are not enough to provoke balancing, what does? I think the notion that Japan could be provoked into balancing by the DPRK rather than China certainly has merit. A few initial thoughts as to why this may be the case:

1) Increased Economic Dependence: China’s military modernization has been and will continue to be fueled by its growing economy. Japan has become arguably China’s most important economic partner (both in terms of trade and investment) over the past few decades. With Japan being China’s third largest export market it would seem that the PRC would have less incentive to militarily threaten the Land of the Rising Sun. There is no such interdependence with the North Koreans. Wait, you might say, Japan does provide a ton of aid to the DPRK. Surely that can create a form of dependence that would deincentivise military provocation. Except that historically it hasn’t stopped the DPRK from continuous provocations. And Japan has repeatedly suspended aid in the wake of missile and nuclear tests.

2) Provocative Signals and Established Images: North Korea has repeatedly test-fired missiles in Japan’s direction, recently test-fired a ballistic missile over Japan, has a history of naval incidents with the Japanese, and as is well known recently conducted an underground nuclear test. Taken together, these are recent provocative signals that make it more likely Japan will see North Korea as a threat. At the very least, it makes it much harder for Japan to comfortably predict status quo behavior from the DPRK. There hasn’t been much for Japan to use to build a status-quo image of the DPRK in the last few decades, meaning most actions by the North are likely to be interpreted as evidence of their hostile and revisionist nature. Simply reviving aid will not be enough to reliably predict status-quo behavior going forward.

3) Domestic Politics: Over the past few decades there has been a growing call with Japan to re-examine its role internationally, particularly with regards to military affairs and the projection of power. At a minimum, many have called for greater participation in collective defense, which by definition of late has meant the ability to project power and not merely defend the home front. International events can create “windows of opportunity” for domestic policy entrepreneurs looking to alter the status-quo. Various scholars, including so-called “neo-classical realists” focus on the influence that domestic political players can have in shaping a state’s foreign policy.

For me, the two most important factors related to reactionary balancing (as opposed to long-term balancing which does not require a catalytic event) are uncertainty and domestic politics. The role of uncertainty in international politics (and social life in general) cannot be understated, and has certainly been highlighted by scholars from various paradigmatic points of view. The fact that the DPRK isn’t as tightly interwoven and dependent on Japan’s market as, say, China combined with their repeated and recent provocations which bring about detrimental sanctions from the Japanese (in the form of cutting of food aid, etc.) may lead Japan to view the North as unpredictable (or, possibly as predictably hostile). Combining unpredictability with a track record of hostility towards Japan as well as significant military capabilities will likely lead Japan to perceive the DPRK as the more significant threat.
Secondly, and building on the first point, domestic politics is always lurking. Yes, Japan was humbled and restrained as a military power after World War II and Article 9 constrains their ability to project power. However, there are significant parts of the Japanese body politic that have and continue to push for ‘normalization’ regarding their military, whether that be conventional or nuclear. In fact, the lifting of the rhetorical taboo on these topics as been steadily declining for years, and calls for revision has not been limited to right-wing circles. Proponents of revisionist policies often need a catalyst, an opening to allow them to push through a major change in policy. Given the continuing normalization in relations with China, economic integration, and the lack of bold, provocative military signals from China of late makes them a less likely candidate to supply the kind of ‘perturbation’ necessary to bring about change in policy.