In fact, much of this departure in Britain's official discourse about Iraq is little more than political rhetoric designed to cultivate the fig-leaf that will be needed for an eventual withdrawal that might have to take place irrespective of the actual situation inside Iraq. In this respect, the high profile of the British forces in Afghanistan could be a bargaining-chip for the Westminster government; it makes it possible to sell the line - especially to the Bush administration, in a way that avoids an open breach with Britain's chief ally - that two substantial deployments at the same time are beyond Britain's military capacity, and that the severity of the Taliban challenge makes it the priority.

Not that this would impress the Bush administration, which regards the British forces as less important in their role of controlling Basra and surrounding provinces than in keeping the roads from the coast towards Baghdad open. The most likely scenario is that a smaller British contingent - probably less than half the current level - will be kept in Iraq for some time to come, engaging in fewer patrols of urban areas in order to avoid casualties. Tony Blair can present this to the domestic public as both a significant decrease and the first of a two-stage process, while seeking to convince Washington of his continued loyalty.

Two choices and a disaster

Meanwhile, the domestic scene in the United States is also fluid in the run-up to the 7 November congressional elections. A CNN poll suggests that only 20% of Americans think the war is being won, barely half the figure for a year ago. The Bush administration is responding with a mixture of defiant speeches from the president himself, reinforced by the weird spectacle of a cluster of rightwing talk-radio hosts broadcasting their support from a marquee on the White House lawn.

In Baghdad itself, US ambassador Zalmay Khalilzad has said that Iraq can be stabilised within a "reasonable" timeframe, while the US military commander General George Casey has claimed that Iraqi security forces will be in a position to take responsibility for security throughout the country within twelve to eighteen months. Much of this too is political rhetoric designed to serve until after the elections. Such views contrast markedly with the assessment of the army chief-of-staff, General Peter Schoomaker, that the United States plans to keep 140,000 troops in Iraq for the next four years. Indeed, there is even talk of troop reinforcements being needed (see John F Burns, "General May Call for Increase in US Troops in Baghdad", The New York Times, 24 October 2006).

But the open discussion of possible military failure in Iraq can no longer be concealed (see Leslie Gelb, "Would defeat in Iraq be so bad?" Time, 15 October 2006). In this context, it is worth recalling that the wider purposes of US involvement in Iraq make a substantive withdrawal from the region unlikely in the extreme. The last column in this series pointed to the aspiration that underlay the 2003 invasion - a free-market client state in Iraq, obedient to Washington's interests and with a sufficient American presence at four permanent bases to maintain US influence and ensure the survival of an Iraqi government (see "New frontiers: from Iraq to outer space", 19 October 2006).

This outcome in Iraq was considered all the more desirable because of the uncertainty surrounding the stability of the House of Saud and the presence of that notorious rogue state - Iran - across the Persian Gulf. Indeed, the fundamental importance of Gulf oil over the next three decades or more meant that securing Iraq (in view of its location between Saudi Arabia and Iran as well its own oil) was the key to US policy success in the region. The fact that nearly two-thirds of the world's oil can be sourced to the Gulf area, and with China destined to be almost as thirsty as the United States for its oil in the coming period, made American military dominance in the region utterly essential.

From this starting-point, a situation in which Iraq went its own violent way (either as a new jihadi base or as effectively a client of Tehran) was, and is, unthinkable. It follows that with all the talk of diverse options, there are really only two choices for the United States in Iraq - and a fallback "plan C" possibility if catastrophe should ensue.

The first choice is to continue the present campaign, perhaps reinforcing US troops if resources permit, in the hope that the insurgency will eventually wither away. All the indications are that this hope will not be realised, and that the United States will pay a high cost in waiting for it to do so.

The second choice is to abandon Iraq's cities and consolidate US forces in a handful of heavily fortified military bases. The assumption would be that some kind of political accommodation will emerge in Iraq - possibly involving an autocratic regime - which would be obliged to accept long-term US influence based on sheer military power.

In some Washington circles this may seem an attractive second-best strategy, even if a permanent US presence in Iraq would be a target of jihadi paramilitaries and al-Qaida leaders. But in any case it may not prove tenable, and this would put the third possibility on the table: wholesale US withdrawal.

In terms of the fundamental need to maintain control in the Persian Gulf region this would be a foreign policy and security disaster for the United States greater in scale than Vietnam. This does not affect the near-certainty that people in the inner reaches of the Pentagon are thinking hard about the US's options after a retreat from Iraq.

Into the long war

The most likely form a withdrawal would take is the build-up of US forces in four other Gulf states: Kuwait (with its US army and air-force bases), Qatar (with its Centcom regional headquarters), Bahrain (with its US fifth-fleet headquarters) and Oman (with its major US air bases). Djibouti too would become much more significant, as would the more distant base at Diego Garcia.

In military-strategy terms, the focus of US power would shift from the army towards the navy, air force and the marines. This would be a humiliation for the US army - another reason why senior commanders will want to "stay the course" in Iraq.

But there would most definitely not be a wholesale US military withdrawal from the Persian Gulf region, which will remain the most important region in the world from Washington's perspective for at least a generation. Even if an utter fiasco ensues in Iraq, the Pentagon would hang on in the region and seek to use other methods to exercise influence and control.

This "solution" will, however, create a new, double-edged fault-line for the United States. Its first aspect is that any major US presence at the heart of the Islamic world in the region will remain a gift to al-Qaida and other jihadi groups. These groups will also be invigorated by the fact that the US's repositioning was the result - or could plausibly be presented as the result - of their victory in Iraq.

The second aspect is that a wholesale US withdrawal from Iraq will leave Iran as the main regional power. This would be unacceptable to Washington and would set the scene for a long-term confrontation.

An earlier column in this series, written just as the Saddam Hussein regime was collapsing in early April 2003, suggested with some temerity that a thirty-year war was in the offing ("A thirty-year war", 4 April 2003). With the extraordinary prospect of an American defeat in Iraq now possible, the only adjustment to that outlook is that such a decades-long conflict may not be restricted to Iraq but might now come to embrace the wider region.