The tide changes for Obama on Afghanistan

President Obama has told the American people that a difficult decade of war is reaching a conclusion: "These long wars will come to a responsible end" he said, adding: "The tide of war is receding."

It is an oddly poetic phrase echoing Shakespeare's Brutus "There is a tide in the affairs of men. Which, taken at the flood, leads on to fortune."

Prosaically that means you have to snatch your chances when you can, before it is too late.

This is a moment when President Obama can do what was unthinkable two years ago. He can defy the Pentagon. Early in his presidency it would have been too risky to ignore military advice. He had to allow them to have a go at winning their way in Afghanistan.

Even so it is tricky to reject the advice of both your defence secretary and his top general in Afghanistan. So lucky that this is the moment that the former is retiring and the latter changing jobs.

He still couldn't have done it without a shifting tide in public opinion. Polls suggest an increasing number of Americans want to quit Afghanistan asap.

The concerns of voters are weighing heavy with an election around the corner in 2012

The seas do not change so much that Republicans would bathe the president in love.

John McCain has said: "I am concerned that the withdrawal plan that President Obama announced tonight poses an unnecessary risk to the hard-won gains that our troops have made thus far in Afghanistan and to the decisive progress that must still be made."

Tim Pawlenty and Mitt Romney have made similar remarks. But it is a sign of the times that they may be stranded, apart from some of their fellows.

Another candidate, Jon Huntsman, has said the withdrawal is too slow: "Now it is time we move to a focused counter-terror effort which requires significantly fewer boots on the ground than the President discussed tonight."

The president can also argue that the tide really has turned. Bin Laden's death is what allows him to make the case but intelligence agencies do seem to think al-Qaeda is a busted flush and Afghanistan is certainly not a safe haven anymore.

Some argue a terrible attack may well happen in the future. But its would-be perpetrators are more likely to be based in Yemen or the UK than Afghanistan.

But the flood that washes away all other arguments is the state of the economy.

With the other main news of the day a worrying report from the Federal Reserve, a key part of Obama's short speech was when he said: "Over the last decade, we have spent a trillion dollars on war, at a time of rising debt and hard economic times. Now, we must invest in America's greatest resource - our people.

"America, it is time to focus on nation building here at home."

With an election around the corner, voters concerns and fears weigh more heavily than wishes of the generals to pursue elusive victory.