It's difficult to know which country is in more of a tizz, Pakistan or the US, following the arrest of an American "diplomat" for shooting dead two Pakistanis in Lahore last week. It is desperately embarrassing for both and could not come at a worse time – just as the US needs all the help in get from Islamabad if it wants to start bringing home its troops from Afghanistan later this year. But now the diplomatic spat caused by Raymond Davis threatens to further undermine an already awkward alliance.