EFFORTS BY Iran and North Korea to acquire nuclear weapons have been at the forefront of diplomacy and international concern over the past few years, and justifiably so. Neither country has been convincingly stopped, although Iran is negotiating. Elsewhere, though, there has been progress toward preventing nuclear materials from falling into the wrong hands.

The Nuclear Threat Initiative (NTI), a nongovernmental organization, has just published the second edition of its global index on the security of nuclear materials, prepared with help from the Economist Intelligence Unit. The report shows that, since the first study was released in 2012, seven nations — Austria, the Czech Republic, Hungary, Mexico, Sweden, Ukraine and Vietnam — have removed all or most of the nuclear weapons-usable material from their territory. This brings the number of countries with more than a kilogram of such material down to 25, compared with more than 50 states two decades ago. The index also shows that 13 states with more than a kilogram have reduced their stocks in the past four years, including Russia and the United States.

This kind of incremental progress often escapes wide attention; giving up such nuclear materials as highly enriched uranium and plutonium is not often accompanied by a parade. Some of the impetus has been the regular nuclear security summits that began in 2010, where leaders made commitments, and many carried them out. The next summit is in March in the Netherlands.

Before anyone heaves a sigh of relief, a lot still needs to be done. The four nations at the bottom of the NTI’s index present familiar and continuing concerns: North Korea is dead last, followed by Iran, India and Pakistan. India and Pakistan, both of which are increasing their stocks of nuclear materials, are still a particularly worrisome flashpoint. Despite its rank on the index, Pakistan was praised for improved laws and regulations that tighten on-site physical protection. However, the study pointed out the dangers of Pakistan’s instability, corruption and “the presence and capability on its territory of criminal or terrorist groups interested in illicitly acquiring nuclear materials.”

Perhaps the most important finding in the study is connected not to any single nation but rather to the lack of a global system of oversight. About 2,000 metric tons of weapons-usable nuclear materials are spread around the world, yet “there is still no effective global system for how nuclear materials should be secured,” no common global standards or best practices, nor a way to hold nations accountable, according to the study. Former senator Sam Nunn, a co-chairman of the NTI, pointed out that strict standards exist in other high-risk enterprises, including aviation, but not for nuclear materials security. The International Atomic Energy Agency has a role to play but is limited by its mandate and resources.

President Obama once pledged to secure all vulnerable nuclear materials in four years. The deadline won’t be met — this is a long game and a long slog. But the goal remains worthy.