General Stanley McChrystal, the coalition commander in Afghanistan, yesterday gave a convincing pitch that without more troops, soon, the eight-year war could be lost. “The situation is in some ways deteriorating,” he said. “This effort will not remain winnable indefinitely. Public support will not remain indefinitely.”

McChrystal, in Britain to make that pitch privately to politicians and military before yesterday’s speech at the International Institute for Strategic Studies, is the kind of sophisticated communicator and articulate strategist who sometimes emerges from the top of the US Army. Some of them even run for the presidency. He has arrived here after a summer of heavy casualties and rising public alarm, while Britain, like the US’s 40 other coalition partners, waits to see if President Obama will buy the call for more forces.

It’s hard to see how Obama won’t. McChrystal’s much leaked assessment is, as the general says, “as blunt as I know how to be”. It says the US “lagged” in the years while the Taleban regrouped. In calling for more troops, to stop the slide, it doesn’t encourage thoughts of a Plan B.

But there is a worrying aftertaste, a muddy sense of impossibility that comes from his emphasis on the unfathomable complexity of Afghanistan. It did not help that his speech avoided any mention at all of President Karzai, the recent disputed elections, or the drug trade, all central to the coalition’s dilemmas. Asked about narcotics, he made a thoughtful case that it was more damaging in fostering corruption than in funding the Taleban. But asked about Karzai, whose team is surrounded by allegations of fraud, he said merely that “he operates in a complex environment”. After this intelligent respect for the difficulty of the problem, McChrystal didn’t show that he could define a mission that was achievable.

It’s unfair to hold his own joke against him, yet he began with an unsettling quip: that his aim was to speak so convincingly that there would be only applause at the end, no questions. But he’d be happy to answer those, he added, “if that plan fails, and most of mine do”. There’s such a thing as taking humility too far if you’re a general deployed to rescue a disintegrating mission.

Humility, however, is his guiding theme for approaching, as he puts it, a society at constant war for 30 years. He told a devastating anecdote of wanting to build a well “for altruistic purposes”, not realising how it would destabilise the balance of power within a village. Every time someone came to him, raising a forefinger and saying: “This is what you do,” he thought: “You don’t understand”.

Of course, he’s right. Who wouldn’t grant him that point, and gratefully? But he also made a case that he may not have intended: that it’s all but impossible to act and be sure of the consequences. Similarly, his prescriptions imply a level of immersion in Afghan culture that seems out of reach for foreign forces, even if they stayed for ever, and they can’t. He wanted to get young soldiers who’ve been trained to protect themselves and their vehicles to talk to villagers, not to point guns at them as they ride through. That is a radical change, and demands language and cultural skills few will have.

His best point, not much more than an assertion, but powerfully made, was that “Afghanistan is worth it”, because of the threat that it still posed, and for the sake of its people. His second-best was the need for more troops. His weakest was what, exactly, he thinks those troops should do.

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