On taking office, President Obama made an early signal of his wish for improved relations with Russia. He has the opportunity to advance that goal today when he arrives in Moscow to meet Dmitri Medvedev, his Russian counterpart. There are important issues on which the two sides are far from agreement, however. These include Nato expansion, ballistic missile defence, Afghanistan and Iran. Mr Obama is unlikely to revise the US position. Nor should he.

When Mr Obama and Mr Medvedev met for the first time, in April, their joint statement spoke of common interests and declared an intention to co-operate on various issues. Their most obvious shared concern is the reduction of nuclear arms. Mr Obama is right to focus on that issue.

The Strategic Arms Reduction Treaty, signed in 1991, will expire at the end of this year. The Moscow Treaty on Strategic Offensive Reductions, signed in 2002, pledged that the US and Russia would reduce operational strategic nuclear warheads to between 1,700 and 2,200 (that is, by about a third) by 2012. Modest decreases below that ceiling have since been negotiated. Mr Obama should aim lower. Whatever the precise number, the US will remain a nuclear-armed power that, through Nato and other regional alliances, guarantees the collective security of its allies. That role is vital for the preservation of liberty. It is not impaired by reductions in America’s nuclear arsenal. An agreement to reduce strategic and tactical nuclear warheads to somewhere closer to 1,000 than 2,000 would preserve deterrence. The US would retain a range of nuclear options against different targets. No adversary could rationally doubt the ability or willingness of the US to respond to a nuclear threat.

The difficulty for Mr Obama will be if Russia insists on linking nuclear arms reduction to concessions on US ballistic missile defence and the expansion of Nato. Mr Medvedev wants the US formally to abandon the missile defence system that it plans to base in Poland and the Czech Republic, and to renounce the possibility of Nato membership for Ukraine and Georgia. That is not realistic diplomacy.

The most worrying military development is the prospect of a nuclear-armed Iran. If Russia were to co-operate more forcefully in curbing a nuclear programme that is obviously not designed purely for generating electrictity, then there would be a good case for the US to compromise on the timing of missile defence and Nato expansion. But there is no reason to compromise on the principles involved. The missile defence system is not directed towards Russia: it is designed to counter an emerging nuclear threat from the Middle East. That threat is not imminent but it is inevitable on current trends. If Russia wants American co-operation on missile defence, then it must support far more vigorously multilateral efforts to halt the proliferation of nuclear weapons to extremist regimes. It must also co-operate with Western efforts to defeat the activities of the Taleban in Afghanistan.

The expansion of Nato is a vexed issue. The erratic conduct of Georgia’s leadership last summer has lent weight to the case for caution. But the principle of expansion is right. Nato’s reach towards the countries of the former communist bloc has progressively strengthened Westernising forces in each successive category of potential member states. Declaratory support is not enough. Institutions matter. Nato and the European Union are vehicles for advancing democratic values. Russia merits no veto on their expansion. In negotiating with Moscow, Mr Obama must not subordinate the principle of Nato expansion to the short-run requirements of amity.