Secretary of State John F. Kerry, center left, puts his arm around Japanese Foreign Minister Fumio Kishida, center right, after laying wreaths at Hiroshima Peace Memorial Park and Museum on Monday. (Jonathan Ernst/Reuters)

IN THE years immediately after World War II, only a true optimist would have predicted that in the following seven decades nuclear weapons would never again be used on an enemy. Despite close calls and dangerous proliferation, the world has managed that feat. If President Obama chooses to become the first sitting U.S. president to visit Hiroshima, his goal should be to note that achievement — and lay out the work remaining to ensure another seven decades of nuclear peace.

The bombings of Hiroshima on Aug. 6, 1945, and of Nagasaki three days later remain one of the most horrific chapters in the history of warfare, unleashing merciless blasts of heat and radiation. The atomic bombs brought an end to the war with Japan but also ushered in a nuclear age that left all in dread of another mushroom cloud.

Survivors of the attacks, and the Japanese people more broadly, resolved to turn the horror into a warning, and they have borne witness ever since to the uniquely barbaric nature of nuclear arms. It would be fitting for Mr. Obama to pay tribute to their dedication. He can do so without passing judgment on President Truman’s decision to use the bomb, and without interfering in Japan’s internal debate over the proper role for its non-nuclear military establishment in an increasingly dangerous region.

After the Soviet Union obtained nuclear weapons, the era of mutually assured destruction — MAD — began. Deterrence worked during a long, tense Cold War, but not without many errors and false alarms, frighteningly overstocked arsenals, proliferation to other nations and the threat of nuclear materials falling into terrorists’ hands. Now it is past time to end some Cold War practices, such as keeping U.S. and Russian intercontinental ballistic missiles on alert, ready to launch in a matter of minutes. The United States is not carrying out nuclear explosive tests, but ratification of the Comprehensive Test Ban Treaty, rejected by the Senate in 1999, deserves another chance in light of advances in computer-powered simulations.

The reality is that nuclear weapons are not going away soon. They are woven into the fabric of the Atlantic alliance and the security umbrella the United States extends to allies Japan and South Korea. But at Hiroshima, Mr. Obama could examine the unfulfilled ambitions of his Prague speech in 2009, a nuclear agenda that brought him the Nobel Peace Prize, and describe how arsenals could be reduced. The agreement to head off Iran’s nuclear program needs to be monitored vigilantly — and matched with respect to North Korea’s rogue program. Russia and the United States possess the largest nuclear arsenals on the planet; verifiable, binding agreements to reduce their size should remain a goal. The Nunn-Lugar program showed it is possible, working together, to lock up stray nuclear materials. Even China, long secretive about its nuclear programs, has been showing new interest in cooperation on nuclear security.

Mr. Obama might fear that critics will distort the meaning of a trip to Hiroshima. But his presence and his words would draw attention to the difficult challenges ahead. He also could counter the reckless remarks of Republican presidential candidate Donald Trump, who suggested that Japan and South Korea might consider starting their own nuclear weapons programs. Seven decades without a nuclear weapon being used in combat or terrorism is remarkable; it will take dedication to ensure this record continues.