The new US-Russian atomic weapons treaty, hailed by both sides as a historic step in arms control, focuses on relics of the past and not the suitcase bomb or other devices that inspire today’s nuclear nightmares.

That is why President Barack Obama is convening on Monday an extraordinary assembly of world leaders to seek a common strategy for keeping radioactive materials and nuclear components out of the hands of terrorists. His goal is to secure all vulnerable nuclear material around the world within four years.

The New START treaty, which replaces the expired Strategic Arms Reduction Treaty of 1991, doesn’t deal more than indirectly with another of today’s big risks: the spread of nuclear know-how to hostile states like Iran, North Korea and perhaps others that might feel compelled to go nuclear, if only in self-defence.

Sen Joe Lieberman, I-Conn, said Thursday that regardless of the merits of the arms treaty, he worries that “we are losing the real world fight to prevent rogue regimes like Iran” from getting the bomb.

Obama will attend a conference in New York next month for a review of the Nuclear Non-Proliferation Treaty, to close loopholes in the internationally recognised rules against the spread of weapons technology. Arms control experts say the New START treaty burnishes US and Russian credentials for insisting other countries forgo atomic weapons, since it demonstrates a commitment to disarm.

The treaty sets a limit of 1,550 strategic nuclear warheads for each side, down from 2,200 under a 2002 deal. The pact also re-establishes anti-cheating procedures that were not written into the 2002 accord, thus providing the most comprehensive and substantial arms control agreement since the 1991 treaty.

At the signing ceremony with Russian President Dmitry Medvedev in Prague, Obama acknowledged that the nature of the peril posed by atomic weapons has changed since the Soviet era, when nuclear arsenals were growing and the world’s biggest fear was all-out nuclear war.

“Nuclear weapons are not simply an issue for the United States and Russia,” he said. “A nuclear weapon in the hands of a terrorist is a danger to people everywhere — from Moscow to New York, from the cities of Europe to South Asia.”

So while the missiles that the US and Russia could hit each other with in a matter of minutes are an obvious danger, the passing of the Cold War, the rise of radical Islamic extremism and the emergence of sophisticated terrorist networks has changed the nuclear equation.

Gen James Cartwright, vice chairman of the Joint Chiefs of Staff, put it in a nutshell last year when he told a missile defence conference that the big risk is no longer being targeted by ballistic missiles.

“Ballistic missiles are about as passe as e-mail,” he said. “Nobody does it anymore.”

He had in mind, as an example, al-Qaeda with its stated goal of obtaining a nuclear bomb — perhaps a small device it could sneak into a US city and detonate with catastrophic effect.

Rolf Mowatt-Larssen, a former CIA officer who led US efforts following the September 11 attacks to determine whether al-Qaeda had a bomb, says al-Qaeda has undertaken a patient campaign to steal or construct an “improvised nuclear device” with the aim of inflicting mass casualties to change the course of history.

Obama will meet with leaders of more than 40 countries tomorrow and Tuesday with the expectation of issuing a joint statement on the challenges and importance of nuclear security. They also hope to agree on a common “work plan” for cracking down on the illicit trade of nuclear material.

Iran, which will not attend next week’s nuclear security summit, is near the top of the list of what the White House calls “outliers”, suspected of being engaged in the illicit nuclear trade. Although Tehran insists its nuclear programme is intended only to produce electricity, the US and others believe it is secretly developing the capability to build a bomb.

Thursday’s treaty signing and next week’s security summit follow another important advancement of Obama’s nuclear agenda — his release Tuesday of a revised nuclear weapons policy that put less emphasis on the utility of nuclear missiles and more on the danger of nuclear terrorism.

“The greatest threat to US and global security is no longer a nuclear exchange between nations, but nuclear terrorism by violent extremists and nuclear proliferation to an increasing number of states,” Obama said in announcing his policy Tuesday. Those new dangers, he said, are “at the top of America’s nuclear agenda”.