We already walked away, it was 1989.

Financial Times: At a recent meeting with senior military staff, James Jones, Barack Obama's national security adviser, warned that any request for more US troops in Afghanistan would prompt a presidential "WTF" moment. WTF, in the highly acronymic Pentagon culture, literally means "whisky tango foxtrot". But in practice it means: "What the f***?"

Having already "surged" an additional 21,000 troops to Afghanistan bringing US forces up to a peak of 68,000, Mr Obama could be forgiven for responding with expletives. At a time when the US president is under acute pressure to rein in a huge US fiscal deficit and when the Pentagon is severely overstretched, another hefty troop request would be hard to satisfy.

Yet it would be even harder to turn down. Having sacked David McKiernan, the previous military commander in Afghanistan, and replaced him with David McChrystal, Mr Obama is beholden to the thinking on the ground. Gen McChrystal, an expert in the troop-intensive ways of fighting counter-insurgencies, is readying a much-awaited assessment report, which is expected to include a request for at least another 10,000 troops. In fact, there are credible rumours that Gen McChrystal could even be planning to ask for as many as 20,000 to 30,000 more. Any such request would be likely to get the backing of David Petraeus, the head of Central Command, which oversees Afghanistan, and author of the counter-insurgency troop surge in Iraq.

It would also meet with the approval of most experts, who point out that the ratio of troops to population in Afghanistan is too low to guarantee success. The history of counter-insurgencies also suggests it takes 12 to 14 years to succeed. Given the shifting nature of the Bush administration's efforts in Afghanistan, 2009 should be dated as year one of the counter-insurgency, rather than year eight.

All of which will put Mr Obama in a quandary. As the US troop presence grows, so too does the cost of America's efforts in Afghanistan. Last month a record 76 coalition troops were killed in Afghanistan - more than in 2002, 2003 or 2004 and roughly half the level of the intervening years. As US troops fan out into the smaller towns in the new "clear, hold and build" strategy, the fatality rate is likely to rise. At some point, the monthly body bag count is likely to exceed 100 - which could spark a serious backlash in the US.

How will Mr Obama respond? There are no easy exit strategies from the policy Mr Obama has embraced.