Why Cameron ended his US visit at Ground Zero

David and Samantha Cameron with New York Mayor Michael Bloomberg at Ground Zero Photo: Stefan Rousseau/PA Wire

It is all in the timing. Given its iconic importance across America and beyond, it was no great surprise that David Cameron chose to end his trip to the USA with a visit to Ground Zero.

But the developments in Afghanistan today (the Taliban calling off talks and Karzai demanding coalition troops withdraw to their barracks) were not helpful, to say the least.

The Prime Minister's argument standing on Ground Zero (with which you may now be wearily familiar) was that the sacrifices in Helmand and elsewhere do serve a higher purpose; Afghanistan might not be quite the country Western leaders would like it to be, but it is no longer a safe haven for terrorists.

David Cameron walks with wife Samantha while viewing Ground Zero from the 21st floor of One World Trade Credit: John Moore/PA Wire

The trouble is that any student of history will find plenty of uncomfortable parallels.

In the late 1960s, an exhausted and disillusioned America decided that it was going to hand over responsibility for defending the free world against communism in Asia to the forces of the ARVN; the Army of the Republic of South Vietnam.

The argument went then that US ground troops had done enough to beat back the North Vietnamese and train their South Vietnamese colleagues and that the country they left behind should therefore be solid and stable.

It was not and the chaotic departure from Saigon five years later was not pretty.

It is hard to argue now that much was achieved in that intervention, save for the loss of more than 50,000 American lives and those of countless Vietnamese.

Nobody wants Kabul to witness similar scenes. But as things stand, you would not confidently bet against it.