Just as fears of nuclear terrorism are rising, US President Barack Obama's drive to lock down vulnerable atomic materials worldwide seems to have lost momentum and could slow further.
With less than 10 months left in office to follow through on one of his signature foreign policy initiatives, Obama will convene leaders from more than 50 countries in Washington this week for his fourth and final Nuclear Security Summit, a high-level diplomatic process that started and will end on his watch.
Foreign Minister Julie Bishop will be representing Australia at the summit.
A boycott by Russian President Vladimir Putin, apparently unwilling to join in a US-dominated gathering at a time of increased tensions between Washington and Moscow, adds to doubts that the meeting will yield major results.
Deadly militant attacks in Brussels have fuelled concern that Islamic State could eventually target nuclear plants and develop radioactive "dirty bombs", a topic that may well be uppermost in leaders' minds as they meet.
Despite significant progress by Obama in persuading dozens of countries to rid themselves of bomb-making materials or reduce and safeguard stockpiles, much of the world's plutonium and enriched uranium remains vulnerable to theft.
The absence of Russia, one of the biggest atomic powers, could detract from decisions reached in Washington this week.
While noting that Moscow had continued joint work on nuclear security, White House spokesman Josh Earnest said it was going to "miss out on an opportunity" and that its no-show illustrated the "degree to which Russia is isolated". Russia has chafed over US-led sanctions over the Ukraine conflict.
Efforts to make the world safer have also been complicated by North Korea's nuclear weapons advance and Pakistan's move towards smaller, tactical nuclear weapons, which Washington fears may further destabilise an already volatile region.
All of this weighs on Obama's agenda as he prepares to host world leaders on Thursday and Friday. He inaugurated the event nearly six years ago, after using a landmark speech in Prague in 2009 to lay out the goal of ridding the world of nuclear weapons as a central theme of his presidency.
But there is no guarantee that once Obama, the driving force behind the initiative, leaves office in January his successor will keep the issue a high priority.