Japan has territorial disputes with each of its international neighbors in the form of sovereignty contests over small islands that are shards of its once vast 20th-century empire. Recently emerging global ocean laws have taken root that make it in every nation’s interests to lay claim to exclusively controlled ocean space. As a result, a new kind of ocean imperialism is underway, compelling some nations to take maximalist approaches and others more flexible positions toward defining their countries’ respective claims. Since the 1990s, Japanese leaders have made clear that they are collectively committed to national policies and planning that reorient Japan as a maritime nation, which was not the case in the wake of the nation’s devastating losses in World War II. The question now is whether Japanese leaders will adopt a rigid definition for Japan or a more fluid one that emphasizes borderlines in the sea around it.