World leaders declared progress over safeguarding nuclear materials sought by terrorists, even as President Barack Obama admitted the task was far from finished.
Ending a nuclear security summit in Washington, Mr Obama warned of a persistent and harrowing threat from terrorists getting their hands on a nuclear bomb.
He urged fellow leaders not to be complacent about the risk of catastrophe, saying that such an attack by the Islamic State or a similar group would "change our world".
"I'm the first to acknowledge the great deal of work that remains," Mr Obama said.
He accepted the vision of nuclear disarmament he laid out at the start of his presidency may not be realised during his lifetime, adding: "But we've begun."
Despite their calls for further action, the roughly 50 leaders assembled announced that this year's gathering would be the last of this kind.
This year, deep concerns about terrorism were the focus, as leaders grappled with the notion that the next Paris or Brussels could involve an attack with a nuclear weapon or dirty bomb.
"There is no doubt that if these madmen ever got their hands on a nuclear bomb or nuclear material they most certainly would use it to kill as many innocent people as possible," Mr Obama said.
So far, no terrorists have obtained a nuclear weapon or a dirty bomb, he said, crediting global efforts to secure nuclear material.
But it was not for lack of the terrorists trying, as al-Qaida has sought nuclear materials, IS has deployed chemical weapons, and extremists linked to the Brussels and Paris attacks were found to have spied on a senior Belgian nuclear official.
The two-day summit held other, more positive signs of the world coming together to confront the broader nuclear threat.
The UN Security Council members who brokered a sweeping nuclear deal with Iran last year held up that agreement as a model for preventing nuclear proliferation.
On the global front, a strengthened nuclear security agreement was finally poised to take force, extending safeguards for nuclear materials being used, stored and transported while requiring criminal penalties for nuclear smuggling.
The changes were approved in 2005 but have sat dormant awaiting ratification from a critical mass of nations, reached only in the past few days.
Still, frustration over the slow pace of reducing nuclear stockpiles overshadowed the summit.
The absence of key players, especially Russia, further underscored the lack of unanimity confronting global efforts to deter nuclear attacks.
Mr Obama has held four such summits, hoping to advance the disarmament goals he set at the start of his presidency, when he declared in Prague that nuclear weapons were "the most dangerous legacy of the Cold War".
Prime Minister Mark Rutte of the Netherlands said: "This summit is not the end of our quest to make the world safe from nuclear terrorism."
He said the assembled leaders were passing the baton to international organisations: "Should the need arise, I know that everybody here will be ready to return."