JAPANESE VOTERS gave Prime Minister Shinzo Abe and his Liberal Democratic Party (LDP) control of the upper house of parliament in an election Sunday, meaning that for the first time in six years the LDP will have control of both chambers of the Diet. The question is, how will Mr. Abe make use of this opportunity. The United States has a big stake in the answer.

The LDP won control of the more powerful lower house in December, giving Mr. Abe a second chance at the premiership. He has adeptly implemented policies that have Japan’s long-moribund economy moving again. He increased government spending and pushed the central bank to pump up the money supply, both highly popular moves, but he also risked alienating his rural supporters by entering into free-trade talks with the United States and other Pacific nations.

Mr. Abe has said that he needed control of the upper house to implement deeper reforms. His advisers have said that he will take advantage of the end of the “twisted parliament,” as Japanese call divided government, by promoting structural changes, including to the labor market and the farming sector. Many economists view these as essential to sustaining growth, especially as Japan’s population ages and declines, but deeply entrenched interests will resist. The multi-nation free-trade talks, which President Obama has said he wants to conclude by year’s end, could give Mr. Abe some political cover for the difficult changes. In addition, Mr. Abe is likely to push to restart many of the nuclear reactors that were shut after a 2011 earthquake and tsunami. And he will seek to reinterpret Japan’s “peace constitution” to allow its military to engage not only in ­self-defense but also in collective self-defense — to come to the aid of a U.S. ship, say, if it is attacked by North Korea.

These are all divisive issues for Japan and, in the case of constitutional reinterpretation, for its neighbors. But they are nowhere near as divisive as another set of issues that the right wing of the LDP will now push to revisit, involving a reevaluation — a whitewashing, critics would say — of Japan’s behavior in World War II. Despite some personal sympathy for the idea that a winner’s version of history has treated Japan unfairly, Mr. Abe will not expend political capital in this inflammatory direction, his advisers say.

Americans will be hoping that is true. As North Korea flouts international demands to rid itself of nuclear weapons and China increasingly throws its weight around, a healthy U.S.-Japan alliance is the region’s best hope for stability. That alliance, in turn, depends on a prospering Japanese economy and on at least cordial relations between Japan and other U.S. friends in Asia, most notably South Korea. Thanks to Japanese fatigue with the instability of the past decade, and to Mr. Abe’s political skills, he now has the best chance in a long time to deliver on those goals.