An underappreciated complexity in the dispute over the Senkaku Islands is that the merits of the competing claims don’t hinge exclusively on contemporary international law. One of the most central issues is whether the Senkakus were terra nullius when Japan annexed them in 1895. If they were, then Japan prevails on the merits. But in deciding this issue, we’re technically not supposed to look for guidance in recent ICJ opinions, recent scholarly articles and treatises, or recent state practice. The reason is something called the doctrine of intertemporality, which requires that the legality of events be judged in light of the law contemporaneous with their occurrence, rather than with the law in force at the time a dispute is ultimately resolved. The appropriate question, in other words, is not simply whether the Senkaku Islands were terra nullius under the doctrine of occupation, but whether they were terra nullius under that doctrine as it existed in the years leading up to the date of Japanese annexation in 1895. Twentieth and twenty-first century developments in the doctrine are, strictly speaking, irrelevant.

This refinement complicates the analysis in two ways. First, it creates a certain level of unavoidable indeterminacy. There was far less law on the doctrine of occupation back in the nineteenth century. No international tribunals had issued decisions on the matter by that point, and even scholarly treatments were fairly sparse. Once we acknowledge that the contemporaneous authorities didn’t provide substantial guidance on what constitutes effective control, it becomes hard to say with certainty whether China had established effective control over the islands prior to 1895.

A second and more fundamental problem is that it’s questionable whether it’s even appropriate to apply the doctrine of occupation in the first place, regardless of its variant intertemporal manifestations. Today we take for granted that international law applies universally, to all states. But it wasn’t always so. Originating in Western Europe, it took time for this body of law to migrate to other parts of the world. And as recent works by Shin Kawashima and Masaharu Yanagihara demonstrate, that migration didn’t reach China and Japan until the late 1800s. China, for example, didn’t have a single text on Western international law until 1865, and didn’t view itself as operating within the framework of that law until after the First Sino-Japanese War in 1894-1895. To the extent that Chinese officials utilized Western doctrines prior to that time, it was, according to Kawashima, purely tactical: the idea was to “gain[] the upper hand over the West by using the tools of the West.” Similarly, Yanagihara explains that Japan had no knowledge of Western international law prior to Commodore Perry’s arrival in 1853, and that there were no Japanese translations of any relevant texts prior to 1865.

This history carries real significance for the debate over Japan’s early occupation of the Senkakus. At a minimum, it calls into question the legitimacy of using an historically Western doctrine to ascertain the legal consequences of events that took place before the parties themselves accepted the doctrine’s relevance and binding force. As late as October 1874, while locked in a dispute with Japan over the status of Taiwan, China specifically rejected the applicability of the Western concept of terra nullius. In Yanagihara’s words, the Chinese Foreign Ministry asserted that “it was useless to talk about international law in cases concerning Taiwan and China” because “international law ha[d] been quite recently invented in Western countries.” The relevant conceptual framework, in China’s view, was instead the Sino-centric order that had predominated in East Asia for centuries—an order that would presumably resolve uncertainties about the “sovereign” (to the extent such a concept existed) status of proximate “territory” (to the extent such a concept existed) in favor of China itself.

These observations are problematic for an occupation-based Japanese claim to the Senkaku Islands. They don’t, however, do much of anything to a Japanese claim founded on acquisitive prescription, which would rely entirely on effective control that occurred after 1895 and after the two sides had adopted the framework of Western international law. For Japan, this is one of the reasons why acquisitive prescription seems to be a stronger argument than occupation.