David Cameron could not be travelling to Washington in grimmer circumstances. The shocking news  that a lone US soldier killed at least 16 civilians, nine of them children, in a shooting spree on Sunday means that US-Afghan relations have reached a new low.
The more one hears about the story the worse it gets. According to the New York Times, "The man gathered 11 bodies, including those of 4 girls younger than 6, and set fire to them". He may have been acting alone but that will not reduce the risk of reprisal attacks. For many, the massacre will confirm their view that the occupation is irredeemably brutal. It was only three weeks ago that US troops were found to have burned copies of the Qur'an at the main Nato base, an act that led to protests in which six American soldiers were killed.
The dry commitment by General John Allen, the commander of US and Nato forces in Afghanistan, to ensure "that anyone who is found to have committed wrongdoing is held fully accountable" is unlikely to dampen tensions. In a statement on its website, the Taliban has vowed to "take revenge from the invaders and the savage murderers for every single martyr".
A large number from amongst the victims are innocent children, women and the elderly, martyred by the American barbarians who mercilessly robbed them of their precious lives and drenched their hands with their innocent blood.
The American terrorists want to come up with an excuse for the perpetrator of this inhumane crime by claiming that this immoral culprit was mentally ill.
If the perpetrators of this massacre were in fact mentally ill, then this testifies to yet another moral transgression by the American military because they are arming lunatics in Afghanistan who turn their weapons against the defenceless Afghans without giving a second thought.
Coming so soon after the deaths of six British soldiers last week, there is understandable concern that UK troops may be caught in the fall-out. All of which means that the onus is on Cameron to restate the case for Britain's continued presence. As the shadow foreign secretary, Douglas Alexander , notes in today's Guardian, the PM came to office promising that Afghanistan would be his "number one priority", but it is now eight months since he made a parliamentary statement about it. He writes: "As the prime minister heads to Washington my concern is that he has an end date, but still no end state: no realistic vision of what will be left behind. And in these critical months the scale of military sacrifice does not appear to be being matched by diplomatic effort."
With the fighting likely to intensify in the run-up to the 2014 withdrawal date, the question confronting western leaders is what sort of country they will leave behind. For now, it is likely to be one even more blighted by violence, corruption and the drugs trade. Consequently, a growing number of voices on the left and the right contend that the benefits of remaining are outweighed by the costs. It is up to Cameron to persuade them otherwise.