In so doing, he placed the need to bottle up loose nuclear weapons or their spark plugs — the fissile materials that make them go bang — even higher on the list of his priorities than slowing climate change, stopping an Iranian nuclear weapon, or brokering a historic Middle East peace deal.

The issue animates Obama enough to host an international summit about it in Washington on Thursday and Friday — the fourth and final one of his presidency — with heads of state expected from as many as 50 countries. Experts familiar with the draft communiques say the meeting may bring some new, minor progress: A few more countries may offer to give back their nuclear materials. New tasks, such as more studies and security inspections, will be assigned to the International Atomic Energy Agency. And new discussions will be held about the dangers posed by a diversion of nuclear materials into so-called dirty bombs that might spread radioactive contamination.

But as the Obama administration winds down and the final summit nears, it’s a good time to consider whether the president kept his ambitious promise. The broad consensus: Despite some progress, the sweeping goals he articulated not only remain unfulfilled, but out of reach for the foreseeable future.

To be fair, the challenge of ensuring that nuclear explosives are not misused has been with us for decades. But why has such a sensible, important, and urgent goal been so hard to realize? Who’s responsible? How can the world make more, faster progress, before a calamitous nuclear explosion — potentially the first in a populated area in more than 60 years — makes every nation wish they had taken the issue more seriously and acted with more haste?

To answer these questions, in 2013 my colleagues and I at the Center for Public Integrity (CPI) embarked on an investigation into what we called the nuclear security outliers: the recalcitrant states, misguided efforts, and wasted opportunities that have undermined the decades-long, U.S.-led effort to put fissile material genies back into bottles.

The scope of the threat, we learned, is daunting. The world’s military and civilian nuclear programs have produced about 500 metric tons of pure plutonium, an amount that could fuel tens of thousands of nuclear weapons while fitting comfortably in a backyard shed. Countries with nuclear programs continue to add roughly two tons to this inventory every year. And yet, it doesn’t take much to unleash a catastrophe: All you need to build a nuclear bomb is a grapefruit-size bit of plutonium.

Highly enriched uranium that could be fashioned into a bomb (the other nuclear spark plug, besides plutonium) is actually a terrorist’s explosive of choice, because it’s a bit easier to handle and use and there’s more of it around.

Roughly 1,390 metric tons of highly enriched uranium are parked at hundreds of military and civilian sites in two dozen countries. More grim news: A single bomb’s worth could fit into an empty five-pound sack of flour and emit so little radiation that it could be carried in a backpack with little hazard to the wearer. Physicists say a sizable nuclear blast could be readily achieved by slamming two specifically molded chunks of it together at high speed. The majority of this inventory is housed in the United States and Russia, but large stocks also exist in Britain, France, India, Pakistan, China and Japan.

Altogether, the stockpiles could, theoretically, be used to construct 20,000 uranium bombs and nearly 80,000 plutonium weapons. Such a massive stockpile does not bode well for humanity, as investigative journalist Eric Schlosser so eloquently wrote in his recent book about the command of nuclear weapons. It’s a disaster waiting to happen.

A tiny fraction of the overall defense budget

Obama certainly hasn’t been a slacker. He’s spoken powerfully about the nuclear terrorism threat before international audiences, and organized three previous summits to try to persuade or bludgeon other nations into taking this issue as seriously as nuclear weapons experts do.

The summits have been useful stages for roughly a dozen countries to announce that they’ve had their last nuclear bender. They’ve told the world that they either have sent or will soon send all their nuclear materials back to Russia or the United States for safekeeping.

The Obama administration, moreover, has invested more than $5 billion in nuclear security programs, according to the Nuclear Threat Initiative, a Washington non-profit group that advocates for tighter control of nuclear explosive materials. That figure includes funds given to Russia and other countries to help secure their weapons, convert research reactors so they burn fuels that cannot be used in weapons, and improve the physical security and accounting of nuclear explosive materials.

Five billion dollars over seven years is no chump change, but it’s less than 1 percent of the amount spent on national defense during each year of the Obama presidency. During his administration, moreover, he has scaled back his nuclear security goals and settled for what Laura Holgate, the senior White House official working on this topic, described in 2012 as “the incremental nature of success,” rather than throwing his full weight behind the creation of new global security standards that, independent experts say, would have had a more lasting and significant impact.

In May 2013, the Department of Energy’s National Nuclear Security Administration (NNSA) noted that while U.S. initiatives under Obama had made the world safer, there are still “serious threats that require urgent attention.” The NNSA’s report — labeled “Official Use Only” but obtained by the Center for Public Integrity — said that terrorists were obviously still seeking nuclear weapons or the raw materials to build them.

The report also called for removing or eliminating 1.1 metric tons of weapons-grade uranium and over 880 pounds of plutonium from sites around the world. It urged the removal of all highly enriched uranium in eight additional foreign countries by 2016. It proposed that the Obama administration undertake a better accounting of plutonium stocks, choose the best ways to dispose of them, and persuade other countries to balance production with consumption so that the net global stockpile will finally start shrinking. It argued for substantially accelerating U.S. efforts to convert research reactors that use weapons-grade uranium to burn a form of uranium that cannot easily be used to fuel weapons, calling for 13 more such reactor conversions by December.

In the end, neither the Energy Department nor the administration officially adopted these deadlines. Too difficult diplomatically and too costly domestically, the Obama White House decided. Meanwhile, in other countries, “there wasn’t much interest” in the efforts, a former senior official told CPI, and the political benefits for the administration turned out to be more limited than anticipated.

Nuclear weapons modernization gets a higher priority

In 2014, as the administration debated how much to invest in nuclear security, it also considered how best to spend the funds. “Should they provide more money for nonproliferation or more money for weapons? It’s clear that weapons won that debate,” Matthew Bunn, a nuclear weapons expert who worked at the White House, said in June of that year.

Since then, proposed spending has fallen well short of earlier projections. So where do we stand now?

Japan, India, Pakistan, the Netherlands, North Korea, and Britain are increasing their stocks of “weapons-usable” nuclear materials, a circumstance that only exacerbates the burdens of locking them away safely. While four nuclear weapons states — France, Britain, the United States, and Israel — signed a useful agreement at 2014’s U.S.-led “Strengthening Nuclear Security Implementation” summit, Russia, China, and Pakistan declined. (India also did not sign it, but has more recently indicated that it may do so this month.) The agreement asked for little more than a commitment to follow International Atomic Energy Agency nuclear security guidelines.

Along with India and Russia, Japan still plans to build a new energy system based on advanced plutonium-burning reactors. In Japan, the fuel would be supplied by a factory at Rokkasho, which will be the world’s largest for producing plutonium. It has a security system that U.S. experts consider too casual, making its imminent opening a sore point in that country’s nuclear-related discussions with Washington for at least the past decade.

Already, Japan has 9.3 metric tons of plutonium stored at Rokkasho and nine other sites; about 35 tons of plutonium are stored in France and Britain. Once Rokkasho opens, the size of its stockpile could easily double in five and a half years, because by Tokyo’s own forecast, Japan is at least 20 years from completing the first of the commercial reactors designed to burn the plutonium that Rokkasho will produce.

Building such large factories for nuclear materials poses special risks. Experts say the International Atomic Energy Agency will likely be able to track 99 percent of the plutonium as it moves through the Rokkasho plant. While 99 percent might sound good, the plant’s annual output will be so high that a 1 percent error rate translates into roughly 176 pounds of untraceable plutonium a year — enough for 26 bombs. Critics worry, as a result, that the sizable uncertainties will open the door for insiders to attempt to divert the material.

South Korea has expressed a similar interest in producing plutonium to supply reactor fuel, pointing explicitly to Japan as a precedent. Experts are concerned that Saudi Arabia, Egypt, Algeria, Indonesia, Turkey, and Vietnam could also follow Japan’s example (these countries have discussed the idea but have not committed to moving forward). In 2011, meanwhile, India completed a reprocessing plant capable of extracting new plutonium from about 100 tons of spent fuel yearly at Tarapur, north of Mumbai. Another plutonium plant is under construction at Kalpakkam, south of Chennai, which the Nuclear Threat Initiative said “will likely surpass” Tarapur “as India’s largest plutonium producer.”

India’s security precautions at such sites have generally been panned by U.S. officials, who privately rank them below those in Russia or Pakistan, and also say they see little evidence that the country is doing enough to keep potential terrorists at bay. A paramilitary force responsible for guarding all of India’s nuclear sites is short-staffed, poorly trained, and ill-equipped, our reporting revealed. (Since we published a series of articles about India’s nuclear program last year, a debate has ignited in India over whether it should create a more specialized nuclear security force).

China, meanwhile, has criticized Japan’s plutonium plans but is considering building a new civilian plutonium plant that will be roughly the size of Rokkasho at the site of two decommissioned military plutonium plants at the Jiuquan Complex in Gansu Province.

And the United States itself has not exactly set a shining example. A deal with Russia that called on each nation to rid itself of 34 tons of plutonium extracted from retired weapons has foundered: Moscow has proposed to feed its plutonium into reactors that can produce more of it. Separately, the United States made a complete mess of its plan to burn up the plutonium in reactors, with a half-built specialized fuel factory likely to be mothballed in the wake of massive cost overruns and persistent mismanagement.

The United States is its own worst enemy

Washington still talks a good game. But in the course of our research, we discovered a simple but distressing fact: Many countries don’t take the threat of nuclear terrorism as seriously as the United States. Why?

First, most governments don’t deal well with low-probability, high-consequence events, like a terrorist nuclear blast. It’s always easier to kick the can down the road in hopes that this is not a current-generation problem. Moreover, some nations regard the prospect of militants building an improvised nuclear bomb as the stuff of science fiction; their officials don’t understand how easily such a weapon might be made, even by amateurs. They see themselves as unlikely targets and are reluctant to invest substantial sums to curb what they consider a distant threat. It’s not their problem, they say boldly. They also think the United States is a more likely target for resentful extremists than nations with smaller international footprints. Let Washington deal with it, they say.

Second, other countries also don’t bear as large a burden of history as the United States. For decades, U.S. officials sowed the seeds of a potential terrorist-engineered disaster under a program that operated with the avowed aim of helping smaller, non-nuclear weapons-enabled countries build research reactors and embrace a nuclear-powered future. Pursuing a policy that seems astonishingly misguided with the benefit of hindsight, the United States helped spread nuclear explosive materials around the globe.

Of the 35 countries that received an estimated 23 tons of highly enriched uranium under this program — France, Germany, and Canada, mostly — only 15 have returned all they received, with about 6.1 tons remaining at 40 locations in 20 countries, according to a May 2014 Nuclear Regulatory Commission Report.

Third, and in the most worrisome category, still others see nuclear explosive materials as a tool to heighten their international standing — a dark reputation-enhancer, if you will — or as a kind of insurance policy, something that could one day be converted into weaponry if international conditions warranted it.

Japan falls into this category, as our reporting showed. “Inside Japan … there are entities who wish to be able to maintain the ability to produce Japan’s own plutonium,” former Japanese Prime Minister Naoto Kan told us. “They do not say it in public, but they wish to have the capability to create nuclear weapons in case of a threat.”

South Africa also falls into this category, having indignantly rebuffed Washington’s suggestions that it give up a stockpile of nuclear material sufficient to fuel a half-dozen bombs, now locked in a former silver vault at a nuclear research center near Pretoria, South Africa.

South Africa has little plausible use for bomb-grade uranium aside from its chest-thumping value, our investigation showed. It intends not only to keep it, but insists on the right to make more. “Our international legally binding obligations … allow for the enrichment of uranium for peaceful purposes only, irrespective of the enrichment level,” South African President Jacob Zuma said at the 2012 Nuclear Security Summit in Seoul, South Korea.

This is the same demand made by Iran and approved in last year’s deal with that country by the United States and five other world powers. And it’s one echoed widely elsewhere. Although the Obama administration has tried to discourage uranium enrichment everywhere, leaders in Brazil, Argentina, Egypt, Saudi Arabia, Vietnam, Jordan, and South Korea say they see nuclear power, along with the ability to enrich uranium, as their legitimate right.

There’s no question that the Obama administration’s attention to this issue — and, in particular, its successful efforts to persuade a dozen countries to relinquish all their nuclear explosives — has helped diminish the threat as it existed in 2009. But a sizable obstacle remains:

As South Africa’s longtime nuclear ambassador and policymaker Abdul Minty memorably told us when we saw him at his office in Geneva, “People who smoke can’t tell someone else not to smoke.”

Actually, they can. All the world’s nations don’t have equal standing, as we know. But Minty’s sentiment reflects the fact that many developing countries resent being told to give up the same activities that major powers have long undertaken — even if doing the same thing winds up jeopardizing everyone. And after facing stout resistance in many foreign capitals, Washington’s eagerness to undertake diplomatic battles over nuclear security issues has flagged.

But there will be no shortage of urgent tasks for Obama’s successor if he or she looks closely at the nuclear security danger and decides it warrants attention.

Two circumstances could make a major difference and speed the world’s response. One is the detonation of a terrorist bomb containing nuclear fuel or radiological materials — a classic, low-probability, high-consequence event that everyone will say they didn’t see coming and deeply regret allowing to happen. The other is concerted, sustained pressure from the United States — above and beyond Obama’s high rhetoric about a world made safe from nuclear weapons.

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