LET’S SAY North Korea shoots a missile at a U.S. aircraft carrier, and Japan has the ability to knock the missile down before it strikes. Should it do so?

The U.S. sailors aboard the carrier would certainly say yes. Japanese Prime Minister Shinzo Abe thinks so, too. But Mr. Abe says that, under current law and constitutional interpretation, Japan would be unable to act. He wants to change that.

This strange state of affairs dates back to 1947 , when a defeated Japan, under U.S. occupation, adopted an unprecedented “peace” constitution. “Aspiring sincerely to an international peace based on justice and order, the Japanese people forever renounce war as a sovereign right of the nation and the threat or use of force as means of settling international disputes,” the charter proclaimed.

Within a few years, as the Cold War intensified and the United States sought to rebuild Japan as an ally against the Soviet threat, the constitution began to be reinterpreted. Japan now fields a “self-defense” force that is among the world’s most capable militaries. But Japanese courts and politicians have continued to hold that the force must confine itself literally to self-defense.

Mr. Abe said last week that such an interpretation isn’t viable in today’s world. He wants Japan’s Diet, or parliament, to rewrite a series of laws to enable a more mutually useful alliance with the United States. A reinterpretation also would allow Japanese peacekeepers to offer more robust assistance in U.N. missions and Japanese ships and planes to help bolster the defense of nations like Vietnam and the Philippines which, like Japan, are alarmed by China’s growing assertiveness.

The change, supported by the Obama administration, makes sense. But it needs to be accomplished with caution. Many Japanese — a majority, according to some polls — are dubious about the change; they are proud of the “peace constitution” and the special global role Japan has carved out as a kind of pacifist power.

Japan’s neighbors, notably Korea and China, are nervous about the change, too. Given that both were victims of Japanese aggression in the last century, some trepidation is to be expected. Anxiety has been fueled by nationalist politicians and one-sided textbooks, especially in China, where a Communist autocracy nervous about its own legitimacy finds it useful to keep a potential enemy easily reachable on its propaganda shelf.

But Mr. Abe has needlessly stoked those fears. His visit to a Shinto shrine where Japanese war criminals are honored, statements that have seemed to question the extent of Japan’s culpability in the war, and his associations with right-wing politicians whose statements are even more extreme — all of these have made his motives suspect in neighbors’ eyes. That, in turn, has complicated his reasonable quest to turn Japan, nearly seven decades after war’s end, into a more “normal” country.