The Kremlin's message may be the same but there is a markedly different tone under President Medvedev as he tackles his first big foreign policy challenge over America's missile defence shield in eastern Europe.

His predecessor Vladimir Putin issued regular threats to target nuclear weapons against states that hosted the defence shield in a bid to drive a wedge between the United States and Europe.

President Medvedev opposes the project just as strongly, but has replaced aggressive rhetoric with a sense of injury. Washington's deal with the Czech Republic "offends" Russia greatly, he says, like a man who has been let down by an old friend.

President George W. Bush and Mr Putin made much of their close friendship during almost eight years in power together. But the personal chemistry has not altered America's determination to go ahead with the missile shield against vigorous Russian objections.

Mr Medvedev has reason to be upset not only by the fact of the deal to locate a radar system on Czech soil, but by the timing of it as he made his debut alongside Mr Bush at the G8 Summit in Japan.

Russia will interpret this as a deliberate US snub against a novice president, intended to humiliate him on the international stage by underlining Moscow's powerlessness to derail the programme. The remark by Condoleezza Rice, the US Secretary of State, that Russia's reaction was "predictable, if disappointing" sounded more than a little patronising in this context.

Mr Medvedev may have taken a leaf out a former US President's book by adopting Theodore Roosevelt's maxim that it is better to 'speak softly and carry and big stick - you will go far". He limited himself to promising unspecified countermeasures, while pledging that Russia "isn't going to get hysterical".

Given that Mr Putin's reaction often seemed just that, this is already a significant change of approach. So what lies behind it?

The Kremlin has long believed that it cannot change Mr Bush's mind on the need for a defence shield and has focused instead on stringing out the negotiating process in the hope that a new president in Washington will kill the plan.

President Medvedev's measured response suggests that this remains Russia's principal goal. Poland's hard bargaining with the US over its price for hosting the shield's interceptor missiles has bolstered Moscow's confidence.

But his recent proposals for a new pan-European defence pact, although given short shrift so far, suggest that the emerging young generation of Russian leaders wants to find ways to join with the West rather than confront it. A more moderate and modest leadership style in the Kremlin helps to make the Russian bear less scary to its neighbours after Mr Putin.

Finally, however, missile defence may simply be a less urgent issue than the crisis now brewing in the Caucasus over neighbouring Georgia's breakaway republics of Abkhazia and South Ossetia.

Both Russia and Georgia are making increasingly bellicose noises about a possible new war in the region. Moscow is already blaming the unstable situation on US support for Georgia's pro-Western President Mikheil Saakashvili and any outbreak of fighting threatens to escalate quickly.

Georgia views the crisis as evidence of the lengths to which Russia will go to prevent it joining Nato. The military alliance will consider fresh applications from Georgia and Ukraine in December - a development Russia fiercely opposes.

Nato would not be enthusiastic to open the door to Georgia if war was raging on its territory. But it is hard to believe that Mr Medvedev would contemplate war in the Caucasus so soon after taking office as the new "liberal" face of Russia.

There may be hard-line elements within the so-called "siloviki" of military and security service factions that want to box him in, however. These anti-western factions were bitterly disappointed that Mr Putin did not annoint one of their number as his successor in May and it is unclear whether Mr Medvedev has full control over them.

Some may want to back him into a corner with the West by picking a fight with Georgia to destroy his moderate credentials. Russia's future response to the missile defence shield then would likely be far more aggressive than it is today.

Times Recommends

• Please help: maimed children of war want their lives back
• Cabal seizes diamond riches to buy power
• White House gatecrashers got to the president