While China steadily built its nuclear capability in the 1960s, the United States and Japan sought to strengthen their alliance in the field of nuclear defence and deterrence; they established several channels for bilateral nuclear consultations, reconfirmed the US commitment to defend Japan, and studied the bilateral cooperation on a missile defence system – an ABM system – for Japan. In contrast to the accumulation of historical studies on Japan’s non-nuclear policies originating from antinuclear sentiments in its society, the very existence of US-Japan nuclear alliance politics has escaped the attention of historians. At first glance, it may seem that a shared threat perception on China lead the two states to seek to consolidate their alliance. However, this was not the case. By investigating both governments’ policy-making processes and bilateral interactions, this chapter argues that it was multiple perception gaps between the US and Japanese governments about the implications of the Chinese nuclear development that brought about the strengthening of the alliance. This chapter also points out that while both governments sought to enhance the alliance, they were constrained by antinuclear sentiments, and antimilitarist norms more broadly, in Japanese society.