Since taking office in December 2012, Japanese Prime Minister Shinzo Abe has shown himself to be a strong political leader and a keen strategic thinker. Agreement on new U.S.-Japan defense guidelines, scheduled to be reached next week, and a deal with Washington on the Trans-Pacific Partnership will further strengthen his reputation and standing. But Abe’s most prudent geostrategic move is the one that he has not yet made: reconciliation with America’s other close ally in Northeast Asia, South Korea.

The most important factor in Abe’s geopolitical calculus is the rise of China. He, along with many other Japanese, fears that Japan will suffer if China becomes the preeminent power in Asia. In the run-up to the Sochi Olympics in 2014, Abe toyed with the Russia card as a geostrategic game changer that would enhance Tokyo’s strategic position vis-à-vis Beijing. But Russian President Vladimir Putin’s Crimean land grab and the international backlash against Russian meddling in Eastern Ukraine made this move both unfeasible and unlikely, while increasing Putin’s dependence on China as Russia’s only alternative to western isolation.

Abe has touted values-based ties with a “democratic diamond” of like-minded partners such as Australia, India, and the United States as a way to shape China’s rise. This strategy also may have its limits; China successfully objected to and blocked quadrilateral cooperation when Abe pursued the idea during his first term as prime minister in 2007.

This leaves South Korea as Abe’s best strategic move. Japan’s first National Security Strategy published in December 2013 acknowledged Seoul as a potential strategic partner to Tokyo. As a “dagger pointed at the heart of Japan,” the Korean peninsula has always been essential to Japan’s security. But the legacy of Japan’s colonial and wartime past continues to hold back Korea-Japan relations even though the two countries share interests, values and Washington as their ally and main strategic partner. The shadow of that legacy looms larger in this year that marks both the seventieth anniversary of the end of World War II and the fiftieth anniversary of the normalization of ties between Japan and South Korea.

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China has sought to capitalize on Japan’s incomplete reckoning with history to woo South Korea into its corner. Chinese President Xi Jinping proposed to South Korean President Park Geun-hye last July a joint commemoration of the end of World War II, which Park politely declined despite her efforts to improve Sino-South Korean relations. Instead, South Korea has increasingly turned to the United States as it grows more disturbed by Japan’s perceived backsliding on issues such as upholding past apologies or questions regarding the involvement of the Japanese military government in organizing sex slaves during World War II.

The United States, while taking a clear position on specific war atrocities, insists that it cannot mediate between allies over history. That hands-off policy is increasingly untenable as the rebalance to Asia anticipates greater cooperation among U.S. allies, and a strong and stable Japan-South Korea relationship is key to the rebalance.

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Plainly, what Abe says and does on his upcoming trip to the United States, and especially his speech to a Joint Session of Congress, will have profound implications for not only Japan, but the United States and much of Asia as well. Park and Xi will be listening to his remarks as closely as the assembled members of Congress. The speech must lay out a vision for the U.S.-Japan relationship and need not be consumed with history. If it does more, however, and provides a basis for strengthening Japan-South Korea cooperation during this sensitive anniversary year, then it will strengthen Japan’s strategic position in Asia and burnish Abe’s credentials as a farsighted strategist capable of protecting his country’s national interests.

This piece first appeared in CFR’s blog Asia Unbound here.

Image: Creative Commons 2.0.