So tell me again [2], Dave, why it is that you think Britain should renew Trident?

Are we really happy to say that we'd give up our independent nuclear deterrent when we don't know what is going to happen with Iran, we can't be certain of the future in China?

China?? Does David Cameron really believe that the People's Republic of China is a threat to the United Kingdom? That the Chinese, in the midst of supplying our high-street stores with much of their clothing lines, have prepared military plans to either invade and occupy the British Isles or nuke us to smithereens from afar? And, even if they had, does he think the UK's four Trident-armed nuclear submarines would protect his "big society" from the People's Liberation Army, backed up by 400 Chinese nuclear warheads [3]? It'd be like the Na'vi versus the humans in Avatar [4] - only without a happy ending for the Na'vi.

Random movie references aside, I do, however, have a serious point to make. Cameron is not qualified to be prime minister. The self-professed "heir to Blair", like Tony Blair before him, edges towards Downing Street with little knowledge of the world beyond the white cliffs of Dover. He is, as President Obama is alleged to have remarked, a "lightweight [5]". Labour strategists have smiles on their faces. The Foreign Secretary David Miliband [2] was quick to say that the Leader of the Opposition had issued "an insult to a fellow permanent member of the UN security council and to a country with whom we have just announced a close strategic relationship," adding: "David Cameron should withdraw this slur now."

Brown is fond of remarking that this is no time for novices. Given the state of the economy, and the "fragile recovery", he argues, we have to stick with an experienced leader who can handle crises and has proven judgement. The same applies on the international stage, where uncertainties, threats and conflicts abound.

Can we trust Cameron to handle Britain's foreign policy? He might do more damage than Blair ever did.

This, after all, is not his first gaffe. Last night, he suggested nuclear confrontation with China. In 2008, he implied that Britain, via Nato, would go to war with Russia over Georgia. As I wrote in my column [6] in the magazine, back in January:

Nothing has better illustrated Cameron's inexperience and lack of judgement than his intervention in the South Ossetia conflict in 2008, when he rushed to Tbilisi to declare his support for embattled Georgia, which, he wrongly claimed, had been "illegally invaded" by Russia. However, as the former Tory foreign secretary Malcolm Rifkind pointed out at the time, "Britain, France and Germany are not going to go to war with Russia over South Ossetia", adding that it was "totally unconvincing" to claim that the conflict wouldn't have happened had Georgia been in Nato.

As my colleague James Macintyre and I have long argued, Cameron has been given a pass by the press. But, I'd add, nowhere has that lack of scrutiny been more evident than on the Tories' foreign policy - both in Europe and beyond. Let's see if that changes next week, in the "foreign affairs" leaders' debate on Sky News.