American and Russian negotiators are busy hammering out a new agreement to reduce their nuclear arsenals before President Obama travels to Moscow for a summit with President Medvedev in July.

Mr Obama has set a long-term goal of establishing a world free of nuclear weapons, an ambition endorsed by his Russian counterpart when the two men met in London in April. But Russia will never give up the bomb. Its nuclear-tipped ballistic missiles are all that remains of its claim to be a global superpower on a par with the United States.

The Soviet Union was so decrepit economically that it was frequently described as “Upper Volta with rockets”. Russia’s economy is still recovering from the Soviet collapse and its heavy dependence on exports of energy and raw materials has been exposed by the global financial crisis.

Why, then, would the Kremlin surrender the one thing that gets America's attention? Without the nuclear stockpile Mr Medvedev leads a country with a population smaller than Pakistan’s and a GDP similar in size to Spain’s.

The Bomb gets Russia disproportionate privileges in Washington, a radioactive “special relationship” that, like Britain’s, allows it to punch above its weight in international affairs. Give it all away in some grand bargain and future American Presidents will have little incentive short of politeness to travel to Moscow for pale imitations of the sort of summitry that characterised the Cold War years.

Russia’s status as a nuclear rival to the US also gives it special clout with anti-American regimes. Without the nuclear submarines, warships and long-range bombers, the symbolism of Russian visits would lose their irritant value for Hugo Chavez in Venezuela and the Castro brothers in Cuba.

Even if Kremlin leaders did not enjoy a nostalgic wallow in their superpower past (and they do), Russia has strong security interests in maintaining its nuclear deterrent. It borders China and is within range of Pakistan and India, all nuclear powers that have shown little interest in surrendering their status.

Russia has a historical fear of being overwhelmed from the East, a legacy of the invading Mongol hordes, coupled with a modern demographic crisis that could see its population shrink by a third to 100 million by the middle of this century. The difficulty of hanging on to its vast territory in an era of depopulation is a live concern for Russian leaders.

Vladimir Putin, the Prime Minister, has already suggested darkly that the US and other countries are eager to get their hands on Russia’s vast oil and gas reserves. In the Kremlin’s mind the nuclear trigger keeps them at bay and holds the country together.

So for all of Mr Obama’s fine rhetoric, it will never be in Russia’s interests to implement his vision of a nuclear-free world. The Kremlin enjoys the bargaining process too much to give up the chance to share the limelight with its former superpower adversary.

The best Mr Obama can hope to achieve is a reduction to the minimum level compatible with Russia’s sense of security and international prestige. And that will still be more than enough to blow up the world.

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