When Barack Obama welcomes more than 50 world leaders to Washington on Thursday ahead of his fourth and final nuclear security summit, one of the most important chairs will be empty.

Russia is thought to possess more nuclear weapons than any other country, including the US. Together the cold war foes share more than 90% of the world’s arsenal. So President Vladimir Putin’s decision to boycott the high-level talks threatens to turn them into an elaborate anticlimax, even as fears of nuclear terrorism are on the up.

“This is the last of the summits and Putin isn’t even participating,” said Professor Peter Kuznick, director of the Nuclear Studies Institute at American University in Washington DC. “It’s going to lose momentum at a time we need to be increasing momentum.”

Indeed, one of Obama’s signature foreign policy initiatives is now seen by some as a metaphor for his presidency: laudable for some real and significant achievements but incremental rather than revolutionary, ultimately falling short of ambitious goals, including his own.

Nuclear talks appear to be a victim of the tensions between Washington and Moscow in general and Obama and Putin in particular. Earlier this week, for example, the American president memorably recalled how Putin had disagreed with some of the statements about foreign policy he had made in an interview with the Atlantic magazine. “I pointed out to him, of course, that unlike you, Vladimir, I don’t get to edit the piece before it’s published.”

In an opinion column in the Washington Post on Wednesday, Obama argued that the US and Russia should negotiate to further reduce their nuclear stockpiles. “Our massive cold war nuclear arsenal is poorly suited to today’s threats,” he said. But the Kremlin spokesman, Dmitry Peskov, told Reuters that Russia was skipping the summit because of a “shortage of mutual cooperation” in working out the agenda.

Ben Rhodes, Obama’s deputy national security adviser, described the decision as “a missed opportunity for Russia above all”, adding: “Frankly, all they’re doing is isolating themselves in not participating as they have in the past.” But he insisted that the countries have “ongoing cooperation and dialogue”.

But there is still plenty to talk about in Washington. The recent terrorist attacks in Brussels have fuelled concerns that Islamic State could eventually target nuclear plants and develop radioactive “dirty bombs”. Two of the suicide bombers involved in this month’s attack secretly filmed the daily routine of the head of Belgium’s nuclear research and development programme and considered an attack on a nuclear site in the country, according to Belgian media. There will be a special session during the summit aimed at coordinating efforts to defeat Isis.

Experts warn that much of the world’s plutonium and highly enriched uranium remains vulnerable to theft at thousands of sites, including universities and hospitals. But Laura Holgate, the White House National Security Council’s senior director for weapons of mass destruction, pointed to commitments from 30 countries at the 2014 summit to secure their most dangerous material. “The international community has made it harder than ever for terrorists to acquire nuclear weapons, and that has made us all more secure,” she told reporters.

Then there is the North Korea question, following Pyongyang’s detonation of a nuclear device in January and long-range rocket launch a month later. On Thursday Obama will meet the Japanese prime minister, Shinzo Abe, and South Korean president, Park Geun-hye, followed by separate talks with President Xi Jinping of China. “The international community must remain united in the face of North Korea’s continued provocations,” he wrote in the Washington Post.

North Korea shows off a new rocket launching system. Kim Jong-un has claimed a ‘historic’ advance in the country’s nuclear strike capability. Photograph: KCNA/AFP/Getty Images

Obama inaugurated the nuclear security summit nearly six years ago after a landmark speech in Prague in 2009 that defined ridding the world of nuclear weapons as a central theme of his presidency. He can certainly claim victories, notably the recent Iran deal and 12 countries having completely eliminated highly enriched uranium or separated plutonium, but commentators say progress has slowed since the last summit in 2014 and now Pakistan, India and Japan are moving towards an increase of their stockpiles.

The Nuclear Threat Initiative, an anti-proliferation watchdog, found that over the past two years there has been no improvement in measures including on-site physical protection, security during transport and the ability to recover lost radioactive materials. It said: “The nuclear security summits have had a positive effect, but the strategic goal of developing an effective global nuclear security system remains unachieved.”

Kuznick estimates that on Obama’s watch little more than a ton of nuclear materials has been removed – which still leaves 500 tons of plutonium and 1,300 tons of highly enriched uranium. Of countries that eliminated potential weapons-grade material, he added, some still have nuclear weapons on their soil. “If we want to say this is the best thing that came out of Obama’s Prague speech then I think it’s very disappointing,” he said.

“Any even small step in the direction of securing these highly dangerous materials is a step in the right direction and worth taking. But in the broader spectrum of the entire nuclear threat, the steps have been relatively minor.”

On that day seven years ago in the Czech capital, Obama declared that as the only country to have ever used a nuclear weapon, the US has a moral responsibility to act. “To put an end to cold war thinking, we will reduce the role of nuclear weapons in our national security strategy, and urge others to do the same,” he pledged.

Yet the Nobel peace prize winner has since laid out plans to modernise the US nuclear arsenal over the next 30 years at a potential cost of nearly $1tn. Kuznick said: “I applaud Obama for what he’s trying to do but I think he sends such mixed messages that he undercuts the moral authority of what we’re doing. It’s the height of hypocrisy.”

Other commentators, however, suggest that Obama’s record compares favourably with other nuclear powers such as Russia, Pakistan and Israel. Jim Walsh, research associate at the Massachusetts Institute of Technology’s security studies programme, said: “He deserves credit among the nuclear weapons states for being a leader trying to move things in the right direction. You can’t say that about anybody else.”

Assessing the legacy of the Prague speech, Walsh added: “Would you like to have cleared up all the nuclear materials? Of course. Was it realistic? Of course not. But we got a new start.”

Whoever succeeds Obama – and the prospect of Donald Trump gaining access to the nuclear codes fills some with dread – it is very possible that the issue will be less of a diplomatic priority for the next president. Walsh added: “I’m a fan of the summit process but over time the momentum has slowed. Among those attending there’s a bittersweet feeling this may be the last one for a while. It’s hard to build an international regime in six to eight years.”