SHINZO ABE, who resigned Friday as Japan’s prime minister, can claim some remarkable political accomplishments. He survived in office nearly eight years — the longest term for a prime minister in the country’s history — and accumulated authority unknown by most of his predecessors. He catalyzed a shift away from Japan’s postwar pacifism toward a more proactive defense and assertive foreign policy. And, more than most major U.S. allies, he managed to maintain good relations with President Trump.

For all those reasons, Mr. Abe’s departure, prompted by illness, could deal a blow to U.S. interests in Asia, including efforts to check Chinese aggression and contain North Korea’s nuclear arsenal. Unstable politics produced seven prime ministers in the seven years before Mr. Abe took office, and whoever succeeds him is almost certain to be weaker. Though it seems likely that the successor will be an Abe acolyte, there’s no guarantee that Tokyo’s firmness on North Korea and willingness to join other U.S. allies in standing up to China will be sustained.

Mr. Abe’s most enduring legacy may well be Japan’s adoption of a more muscular defense posture, enabled in part by passage of a law allowing the country’s military forces, which are constitutionally restricted to self-defense, to operate outside national borders. Japan developed new amphibious forces, acquired the largest number of F-35 fighter planes outside the United States, and recently began discussions on whether to purchase missiles that could be used for preemptive strikes. Mr. Abe took a hard line against the North Korean regime of Kim Jong Un, backing Mr. Trump’s initial pressure campaign against Pyongyang and watching in quiet dismay when he switched to personal diplomacy.

At the same time, Mr. Abe’s nationalist instincts prompted him to squander political capital on the long-shot goals of altering the constitution’s pacifist clauses, obtaining the return of islands seized by the Soviet Union at the end of World War II and securing the return of Japanese citizens kidnapped decades ago by North Korea. In a news conference announcing his resignation, he apologized for achieving none of those aims and also acknowledged that he had “received severe criticism” for his handling of the coronavirus pandemic — a display of candor and humility that his sometime golf partner in the White House could learn from.

One consequence of Mr. Abe’s misplaced priorities was a failure to follow through on badly needed economic and social reforms. While the central bank chief he appointed stimulated employment with monetary measures, Mr. Abe largely failed to deliver on the most politically difficult strand of his “Abeconomics,” structural reforms to enhance competition. Mr. Abe loosened restrictions on immigration to allow more foreign workers, but his pledges to expand opportunities for women had little impact on corporate or political life, even as the country’s birthrate and population continued to decline. The challenge of tackling those stubborn problems will now fall to future prime ministers — who, at least in the short term, are unlikely to be Mr. Abe’s political equals.

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