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As he ended the Nuclear Security Summit on Friday, President Obama could claim some success in leading the international community to curb the amount of nuclear materials that could fall into terrorists’ hands.

But even with considerable progress, Mr. Obama has not fulfilled his goal of securing all nuclear materials in four years; some 1,800 metric tons of nuclear material remain stored in 24 countries, much of it vulnerable to theft.

The summit meeting, which drew more than 50 world leaders to Washington, produced many practical commitments to secure and eliminate nuclear materials. Mr. Obama announced that a long-stalled treaty requiring countries to comply with standards for securing nuclear facilities and nuclear material while in storage, use or transit would take effect in 30 days.

Japan and the United States said they had completed the transfer of all highly enriched uranium and separated plutonium from a research project in Tokai Mura, Japan, to a facility in South Carolina where the materials will be converted to forms less suitable for use in weapons. South Korea promised to strengthen nuclear detection at its ports, Kazakhstan said it would strengthen its export control laws and Britain pledged to lead a new cyber security initiative in response to growing concerns that a cyberattack could be made on nuclear plants.

Despite serious tensions with Washington on many other issues, China’s president, Xi Jinping, promised to convert some of China’s nuclear reactors, as well as reactors supplied to Ghana and Nigeria, to low-enriched uranium, which is less suitable for weapons use. The United States also announced plans to look into using low-enriched uranium rather than highly enriched uranium in its naval reactors.

President Vladimir Putin’s refusal to attend the summit meeting, however, was a reminder of how troubled ties with Russia are stymieing Mr. Obama’s ambitious pledge in 2009 to seek a world without nuclear weapons. The two countries still deploy more than 1,800 strategic warheads each.

In 2013, Mr. Obama said America’s arsenal could be reduced by one-third. Even so, he has supported a $1 trillion plan to replace the nuclear arsenal, including bombers, missiles and submarines. Russia is also modernizing its arsenal, while Pakistan, India and China are expanding theirs. Meanwhile, North Korea, despite tough sanctions, is adding to its cache of nuclear weapons and the missiles needed to launch them.

The nuclear security summit meetings have been a useful means of focusing attention on nuclear dangers and persuading countries to be more security conscious. It will be up to the next American president to continue this effort and work to prevent another nuclear arms race.