A neoclassical realist model of overconfidence and the Japan–Soviet Neutrality Pact in 1941

Why did Japan conclude the Japan–Soviet Neutrality Pact in 1941, even though it knew the German-Soviet relations were deteriorating and heading towards a disastrous war? Under the Tripartite Pact, it would be irrational for Japan to approach the USSR, which Germany had decided to invade. This article examines this long-standing puzzle in Japanese diplomatic history (also an anomaly of neorealism) by developing a new neoclassical realist model termed ‘neoclassical realist model of overconfidence’ based on self-deception in evolutionary psychology using scientific realism in the philosophy of science as a metatheoretical foundation. Drawing on neoclassical realism, I argue that Japan’s balancing strategy during 1940–41, which initially reflected the tripolar structure of the international system (independent variable), ultimately resulted in the suboptimal balancing of the Japan–Soviet Neutrality Pact (dependent variable) due to Yosuke Matsuoka’s (Japanese foreign minister) self-deception, including the positive illusion and cognitive dissonance effect (intervening variable).