Just when relations between South Korea and Japan appeared unalterably on the road to perdition, Japanese Prime Minister Shinzo Abe proffered an off-ramp.

Abe defied predictions by affirming rather than rejecting Japan’s previous apologies for its wartime atrocities. His stunning reversal provides an unforeseen opportunity to repair relations between America’s key allies prior to President Obama’s Asia trip next month.

The U.S. has long eschewed assuming a mediator role, given the dangers of being perceived as taking sides on issues that inflame fervent emotions in both South Korea and Japan. But without U.S. shuttle diplomacy, Japan and South Korea will remain fixated on the past, undermining U.S. security objectives in Asia even as the North Korean and Chinese threats increase.

Abe’s bold gesture, due in part to behind-the-scenes stern messages from Washington, is a small yet monumental first step that must be repeatedly affirmed and built upon. But further Japanese actions require reciprocal gestures from South Korea. It is far from certain, however, that President Park Geun-hye is willing to exercise the requisite leadership by defying her country’s fervent nationalism.

Abe had long signaled resistance to postwar descriptions of Tokyo’s war guilt and expressed intent to revise if not repudiate official Japanese apologies. Those earlier provocative comments, along with a December 2013 visit as prime minister to the controversial Yasakuni Shrine, aggravated already frayed Japanese–South Korean ties.

That makes Abe’s recent pledge to affirm the apologies all the more striking and important. For the first time, he declared his administration would uphold the Murayama statement recognizing Japan’s wartime actions. He also declared he had no intention to review the Kono Statement, which acknowledges Japan’s role in procuring “comfort women” (sexual slaves) for the imperial Japanese military.

Knowing the latter issue is of utmost importance to South Korea, Abe provided an additional gesture by commenting, “I am deeply pained to think of the comfort women who experienced immeasurable pain and suffering, a feeling I share equally with my predecessors.”

South Korea responded positively, though cautiously, to Abe’s reversal. President Park had previously defined Abe affirming the official apologies as a prerequisite for a summit with Japan. But Park and other Korean officials now demand additional undefined steps to display Japanese “sincerity.”

Tokyo feels it has shown sufficient contrition for its wartime past, citing a long list of apologies and the passage of time. Japanese media expressed outrage at the U.S. government’s mild public rebuke of “disappointment” after Abe’s trip to Yasukuni. Yet the reality was that Washington was privately seething at Abe’s action. Washington’s ire was exacerbated by public Japanese ruminations questioning the reliability of the U.S. defense of Japan.

Across the Sea of Japan—or the East Sea, as Korean activists would have U.S. textbooks define it—the Park administration has also disappointed Washington by allowing the ghosts of the past to haunt present-day policymaking. South Korean polls show that the public and policymakers are more worried about the hypothetical resurrection of 1930s Japanese militarism than the very real North Korean threat of today.

The depth of South Korean resentment toward Tokyo is embodied in the controversy over South Korea’s military accepting 10,000 bullets from Japan. South Korean peacekeeping forces in Sudan faced imminent threat and called for more ammunition. The only other United Nations force with the same caliber bullets was Japan, which provided the requested assistance.

Yet the ensuing public uproar in South Korea forced the return of the bullets. Apparently South Korea would rather see its own troops placed at risk than show the slightest sign of reconciliation with Japan.

Similarly, South Korean Minister of Foreign Affairs Yun Byung-se used a U.N. human rights conference to lambaste Japan over its wartime actions rather than focusing on the recent U.N. report detailing North Korean crimes against humanity. Yun also advocated forging a military intelligence-sharing agreement with China—which rejects international accusation of North Korean human rights abuses—before finalizing a similar agreement with fellow democracy Japan.

South Korea is more willing to pursue President Park’s trustpolitik policy of mutual trust-building efforts with Pyongyang—which killed 50 South Koreans in 2010 and continues to threaten South Korea with nuclear annihilation—while refusing to engage with Tokyo.

The U.S. should urge Seoul to compartmentalize and prioritize its foreign policy. A myopic focus on history detracts from efforts to address rising security threats to allied national interests. South Korea’s grudging acceptance of a trilateral summit with Obama and Abe on the fringes of the multilateral nuclear summit next week is a proper response, but more needs to be done.

For its part, Tokyo should continue a reconciliation process with Seoul. Abe should repeat his unequivocal affirmation of the official apologies, propose a mutually agreed upon mechanism for compensating surviving women forced into sexual slavery during World War II, and pledge not to visit Yasukuni again.

Failure by Japan and South Korea to use Abe’s gesture as a catalyst to begin a reconciliation process will leave both countries forever hostages to history rather than addressing today’s realities.