AT THE Army College in Fort Leavenworth, Kansas, Major Daniel Morgan is studying the lessons for Iraq of the Soviets’ chaotic exit from Afghanistan in the late 1980s. The roads were choked with tanks and heavy weaponry, making the demoralised soldiers easy prey for guerillas.

“The Soviet Army actually had to fight out of certain areas,” said Morgan, who has served twice in Iraq.

There is no easy exit from Iraq, but defence secretary Robert Gates has admitted for the first time that the Pentagon is poring over the options. Under pressure from Hillary Clinton, the senator for New York, defence officials are to give a “closed door” briefing to the Senate Armed Services Committee this week about US troop withdrawals.

“You may rest assured,” Gates wrote to Clinton, “that such planning is indeed taking place with my active involvement.”

The Soviet rout from Afghanistan is one of the worst-case scenarios that a rapid withdrawal of combat troops from Iraq could provoke. “They had to airlift out of Kandahar, the fighting was so bad,” Morgan recalled. Another nightmare image remains familiar decades on - the helicopters taking off from the roof of the US embassy in Saigon, leaving desperate Vietnamese allies to their fate.

Gary Anderson, a retired Marine Colonel, conducted war-games for the Pentagon in which he played the “Red Commander”, Saddam Hussein, on the eve of the US invasion. He has been invited back to role-play US withdrawal strategies for the defence department in the coming weeks.

“They are starting to think in that direction,” he said. “The Soviet scenario could certainly happen but what really worries me is the example of Vietnam.”

When America lost the will to sustain the South Vietnamese regime, Congress cut off the funding for the war and the South Vietnamese army collapsed.

“I can see that happening to the Iraqi army if this Congress says we’re going to pull out our troops and not support the Iraqis. There would be a political collapse and the very real potential of helicopters on the roofs.”

The alternative, Anderson believes, is for America to implement a phased withdrawal that would bring down the number of troops and increase the number of embedded trainers and advisers. It is the consensus option, favoured by the Iraq Study Group - co-chaired by James Baker - and also Gates and Hillary Clinton though not necessarily by Bush, which could ultimately bring down US forces from 160,000 to 70,000, with roughly 20,000 serving as advisers. But it too carries risks.

The safest way to withdraw, in the words of a Pentagon official, would be to conduct orderly handovers, area by area, to Iraqi forces - “you take down one flag and run up another”. But a study by logistics experts for US Central Command recently concluded that it would take two years systematically to turn over territory, bases and equipment to the Iraqis.

And, as the Soviets found in Afghanistan, once it is obvious the US is on the way out, the political and security situation could rapidly come unstuck.

A classified report by General David Petraeus, the US commander in Iraq, which was revealed in the New York Times last week, suggested that American forces would need to stay in Iraq until the summer of 2009 to ensure stability. In the “near-term”, localised security was to be provided in Baghdad and other trouble spots by June 2008; while the “intermediate” goal was to establish security nationwide in two years’ time. But Washington’s political time-table is on fast-forward.

Admiral William Fallon, the US Middle East commander, said last week, “We have some really big decisions ahead of us. We have to ask ourselves whether the surge is really working and what do we want to do afterward.”

Anderson has already war-gamed the Iraq Study Group’s plans and other options for leaving Iraq. Unlike George W Bush, he doubts that Al-Qaeda will emerge as a major threat postwithdrawal - even though they are likely to harry US forces as they leave.

“I foresee a bad scenario, but it is not the same scenario as the president’s,” Anderson said. “Al-Qaeda is made up of foreigners and our presence is the only reason they are there. Once we leave, they’ll become the foreigners and the Iraqis will turn on them.”

He predicts that the situation will become “ugly” and that Iraq will fracture into its Sunni, Shi’ite and Kurdish regions. However, he doubts that full-scale genocide will ensue. “It will look more like Lebanon in the 1980s than Bosnia - messy, but low level.”

General Jack Keane, a forceful early advocate of the US troop “surge”, said this would be abandoning the Iraqis to their fate.

“It demonstrates a lack of character on the part of the United States,” he said.

Times Recommends

• Tehran cannot live up to its rhetoric
• The new unicycle that will ‘change the world’
• We cannot wish away Iran’s bomb