Leading article: Recognition at last that there can be no military solution in Afghanistan

President Barack Obama's announcement that 33,000 US troops will leave Afghanistan by the end of next year, with the goal of handing over entirely to Afghan forces by 2014, is more than just a turning point in America's longest ever war. It surely marks the end of a post-Cold War era of US interventionism that reached its peak with George W Bush's adventure in Iraq.

After almost a decade of involvement, a change of strategy was inevitable. The killing of Osama bin Laden last month fulfilled the primary goal of the original invasion of October 2001, and as the president noted, the al-Q'aida organisation has already been largely crippled. It is moreover universally accepted that a stable Afghanistan will ultimately be secured only by political, not military, means – by a permanent reconciliation between the government in Kabul and the Taliban. Contacts to that end are now under way.

The war has long been unpopular in Mr Obama's Democratic party, and understandably so. The returns on America's outlays of blood and treasure have been modest at best. Afghanistan is still riddled with corruption, and in many respects the writ of central government extends barely beyond Kabul's city limits. The circumstances of Bin Laden's demise have only hardened a general view that the real enemy is to be found no longer in Afghanistan, but in Pakistan.

The Pentagon would have preferred a slower withdrawal and undoubtedly the drawdown, which over the next 18 months will undo the December 2009 Afghan surge in its entirety, carries risks. US and Nato forces will be stretched thinner; parts of the country now under precarious allied control may slide back into anarchy. The Taliban may be emboldened, while local warlords who have cast in their lot with the US may change their minds.

Nevertheless, the elimination of Bin Laden provides ample cover for his assertions that al-Q'aida was now "on a path to defeat", that "the tide of war is receding", and that "the light of a secure peace" was now discernable. More important, in political terms, the president has little choice but to declare victory and move on.

The outcome of the 2012 election will be decided not by a stagnating foreign war but by the domestic economy, and for Mr Obama the omens are not good. Unemployment is rising again, while just hours before his address on Wednesday evening, the Federal Reserve issued new forecasts, significantly downgrading growth both this year and next. Burdened by $14tn of debt, why should America devote precious resources to rebuilding Kabul and Baghdad, rather than its own stagnating economy? On the campaign trail, the question is unanswerable.

But the mood-change over Afghanistan will outlast 2012. Americans are fed up with long foreign wars. Not just Democrats but large swathes of the Republican party too would have preferred a faster pullout than the one announced by the president.

As last week's Republican candidates' debate vividly demonstrated, hardly a contender has a good word to say about the Afghan and Libyan campaigns. That sentiment is not confined to the Tea Party movement, whose fixation on slashing back government and getting rid of the deficit is driving the Republicans to the right.

Today, the neo-conservative fervour that propelled America into Iraq is almost nowhere to be found. Is this a return of American isolationism, as critics claim? Perhaps. What is clear is that the new mood will last far longer than a single election cycle. That is why Mr Obama's speech marked not just a course correction, but a watershed.