FIRST there was the “open mic” incident last March when Barack Obama assured his Russian opposite number, Dmitry Medvedev, that after his election he would have “flexibility” on the subject of missile defence. Then came the briefest of sentences, in his state-of-the-union address on February 12th, on the need to engage Russia in further reductions to nuclear arsenals. But together they give a clue to what could become a lasting legacy of Mr Obama’s two terms in office: a serious attempt to realise the commitment he made in Prague four years ago when he promised to take “concrete steps towards a world without nuclear weapons”.

Those for whom the Prague speech was a rallying call have found Mr Obama, so far, a bit of disappointment. True enough, he succeeded in getting the Senate to ratify the important but relatively unambitious “New START” strategic arms-reduction treaty with Russia in 2010. The treaty restored on-site inspections while limiting the number of deployed strategic nuclear warheads on each side to 1,550 by 2018—a figure that Russia is already slightly below and which America, with 1,720, is close to achieving ahead of time. It deals neither with non-deployed strategic warheads (America is reckoned to have 2,800 and Russia 1,000) nor tactical warheads (America has around 500 and Russia more than 2,000) stockpiled by both sides.

And not much has happened since. After the quadrennial “Nuclear Posture Review” in 2010, which narrowed the declared role of nuclear weapons in American strategy, the administration has dragged its feet over explaining how it would put it into practice. In 2011 it launched what was meant to be a 90-day implementation study (NPRIS) that would determine how many weapons and of which types America would need to deter attacks on itself and its allies in the light of the changed relationship with Russia since the end of the cold war. But when the NPRIS subsequently leaked, it was temporarily shelved to prevent it becoming a stick for Republicans to beat Mr Obama with during his re-election campaign.

According to Daryl Kimball of the Washington-based Arms Control Association, the NPRIS will be discussed at Mr Obama’s first post-election security cabinet meeting next month. The president is wary of trying to get another treaty through the Senate, so he is contemplating both accelerating the New START reductions and, if agreement can be secured with Russia, moving below the ceiling, perhaps to 1,000 warheads—a figure that the joint chiefs of staff have recently agreed would not put deterrence at any risk.

Tom Donilon, the White House national security adviser, is due to go to Moscow later this month to discuss the proposal. To smooth the rough patches in the present relationship, Mr Donilon may offer to delay the development of new high-speed interceptors in the fourth phase of the European ballistic-missile defence system due for deployment in 2021. Mr Obama is also believed to be keen to “de-alert” his nuclear forces from the hair-trigger, launch-on-warning doctrine that still endures—something he could do with a stroke of the pen.