It must have given David Cameron a great sense of occasion as he addressed the UN General Assembly for the first time yesterday evening, seeking to impress upon them the importance of showing the world's disapproval of brutal dictators through sanctions and military action.

Many will see it as more evidence that the Prime Minister is morphing into Tony Blair, who spoke in Chicago in 1999 about his belief in a "just war" in which the international community acted to stop genocide, ethnic cleansing or humanitarian catastrophe. The speech was the prelude to Nato's intervention to free the Kosovo Albanians from Serbian rule, in which Blair played the lead role. When he visited Kosovo afterwards, he was mobbed by an ecstatic crowd.

For Blair, read Cameron, for Kosovo read Libya. It must have been a heady experience for Mr Cameron to receive the adulation of the crowd in Benghazi last week. Prime ministers are rarely feted like that at home.

The Independent revealed this week that David Cameron has been playing host to Tony Blair at Chequers so that his Labour predecessor can offer advice on world affairs. This is a different Mr Cameron from the one who took office 18 months ago. Then, foreign affairs seemed barely to have engaged his attention. He apparently thought that Iran has nuclear weapons, and he famously described Gaza as a "prison camp". His background is in domestic policy, which is also the battleground on which general elections are won. And he appeared content to leave the field to the Foreign Secretary, William Hague, whose approach was highly pragmatic, as if he believed that British embassies were primarily offices for promoting British exports.

As recently as February, David Cameron told an audience in Cairo, in the heat of the Arab Spring: "I am not a naïve neocon who thinks you can drop democracy out of an aeroplane at 40,000ft." But soon afterwards, British warplanes were flying over Libya, bringing succour to what we hope will be the forces of democracy.

The Libyan experience may not have turned Mr Cameron into a naïve neocon, but it has certainly given him an itch for action against other dictators, notably President Bashar al-Assad of Syria. In those private chats with Tony Blair, he may also have heard the former Prime Minister advocate regime change in Iran. While it is much to be desired that all the cruel, corrupt and anti-democratic regimes be removed and replaced by elected governments answerable to the people, Mr Cameron should beware of catching the messianic disease that took Tony Blair into Iraq.

Britain is not a world power as it was in Gladstone's day. We do not have the military capacity to intervene everywhere that a government is oppressing its people by violence, even if it were desirable that we do so. And in reality, as we all know, the will to intervene is much stronger when the target is an anti-Western demagogue like Colonel Gaddafi than when the crowds are protesting against a friendly government, such as that of Bahrain.

Mr Cameron should also be aware of the recurring temptation for prime ministers facing intractable problems at home to escape them by strutting the world stage. It is so much easier to address an adoring crowd in Tripoli or deliver a thoughtful speech to the UN General Assembly than it is to explain why unemployment and inflation are so high, while public spending cuts are provoking hundreds of thousands of public sector employees to get ready to go on strike. The Prime Minister must avoid using foreign policy as displacement activity when his attention is needed at home.