[...]

In his private discussions with British Defence Secretary Des Browne and with the prime minister, Gen Petraeus will be setting out the rationale behind current US thinking.

But it would be surprising if he did not stress the US view of the need for a continuing UK troop presence on the ground.

This is not just a matter of appearances, though being deserted by its leading ally would have severe political repercussions in Washington.

It is also a matter of practical strategy as well. If the British go, then the US will have to find units from its own over-stretched military to bolster security in the south.

Those supply routes to Kuwait are vital for the Americans too and they also provide the best exit route for any eventual US withdrawal.