AS LONG as American presidents have commanded nuclear arsenals, they have yearned to be rid of them. These weapons “must be abolished before they abolish us”, John F. Kennedy said. Ronald Reagan dreamed of their “total elimination”. In a 2009 Prague speech Barack Obama vowed “concrete steps towards a world without nuclear weapons”.

Speaking in Berlin on June 19th—four years after his Prague speech was cheered, then ignored—Mr Obama tried again, announcing a series of steps towards disarmament. Some are in his gift, eg, limiting the scenarios that would trigger American nuclear strikes. But most require consent from others, from Russia to the Senate in Washington, DC.

Mr Obama repeated in Berlin an offer already made in private to Russia’s president, Vladimir Putin: to reduce both sides’ deployed strategic nuclear weapons beyond cuts agreed in the 2010 New START Treaty. America could live with an arsenal reduced by up to a third, Mr Obama suggested. That would leave each country with just over 1,000 such weapons, if Mr Putin reciprocated.

Mr Obama talked of rejecting “the nuclear weaponisation that North Korea and Iran may be seeking”: a cautious form of words that avoided early confrontation with Iran’s president-elect, the avowed moderate Hassan Rohani. Mr Obama vowed to seek support for American ratification of the 1996 Comprehensive Nuclear Test Ban Treaty (a long shot) and called for a global treaty banning the production of nuclear bomb-making materials (a hopeless task).

Obstacles loom. On the day of Mr Obama’s Berlin speech, Mr Putin grumbled about American anti-missile shields and about new high-precision non-nuclear weapons that he said approached the strike capability of strategic nuclear arms. In essence, Mr Putin thinks American talk of nuclear disarmament is “a plot to take over the world with conventional weapons”, says George Perkovich of the Carnegie Endowment for International Peace, a think-tank.

Senate Republicans attack Mr Obama from the opposite direction. Bob Corker of Tennessee says that trimming America’s arsenal without modernising existing weapons could amount to “unilateral disarmament”, adding that he has been promised by the secretary of state, John Kerry, that further nuclear reductions would involve treaty talks with Russia, thus requiring Senate consent.

Legally, Mr Obama could agree cuts with Mr Putin, bypassing the Senate. But that is politically risky. Mr Obama wants disarmament as a legacy, so he is trying to rally global opinion to the cause. To date he looks unhappily alone.