American voters will ultimately have to decide how many more years - if any - US forces stay in Afghanistan

It is a given that if a newly elected American president inherits a war, he or she will pledge to end it with honour and bring the troops home. Barack Obama, more a peace president by inclination, was no exception. But he has finally conceded defeat more than a year before leaving office after eight years.

His latest retreat is to maintain the 10,000-odd servicemen still in Afghanistan for most of next year before reducing the number to 5,500. He leaves his successor as leader of the world's most powerful nation the toxic legacy of its longest war - 14 years - with no end in sight, let alone a victory.

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Obama was in no small way elected by a conflict-weary country to end the exhausting ground wars of his predecessor, George W. Bush. It was to be a point of difference, with the prospect of closure of kinds on the trauma of the 9/11 terrorist attacks.

However, Bush's legacy has haunted him. But for Bush's folly of attacking Iraq on deeply flawed grounds without UN backing, things might have turned out differently. The invasion and subsequent debilitating quagmire of occupation weakened the focus, post 9/11, on securing the stability of Afghanistan after the rout of the Taliban who, predictably, took advantage to regroup and undermine both.

The result is an unmitigated disaster for the US, Afghanistan and the region. Determined to avenge 9/11, Washington committed so many resources and sacrificed so many lives, yet Afghanistan remains on the brink of collapse, wracked by corrupt government and local warlords.

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If the US leaves, there is a serious possibility the current government could be overrun by rebels. It is a moot point, given the corruption of some of the warlords the US has been supporting, how much worse off many Afghans would be. Militarily there is an argument that the US-led counter-insurgency has failed and that the question now is how to support one that is Afghan-led.

The more immediate and sinister threat may come from the extremist rampage of Islamic State, or IS, with its global appeal to disaffected Muslims. But, thanks to 9/11, the Taliban issue still weighs on the American psyche. Hence the perception that Obama needs to keep troops there to protect Americans.

The fact that 5,500 personnel will be spread extremely thinly over counter-insurgency efforts means Obama's successor will have to do something more decisive sooner rather than later. On that matter, there is a significant unknown: to what degree, if at all, are American voters willing to support the necessary greater military involvement?