David Cameron was off to make a defining speech on Europe when what could prove defining events deep in the Algerian desert threw him off course .
The questions are many. If, as now seems likely, there has been loss of life among the hostages at the oil and gas plant, then Britain and America may well find themselves looking for ways to hit back at these al-Qaeda-linked groups operating in the Sahel region of Africa.
That may lead to greater involvement than they would have wished in the unfolding campaign in Mali.
Will they decide to step up support for the French who have troops on the ground there and seem set for a prolonged fight if necessary?
And will that mean British boots on the ground after all?
And will it mean a broader resumption of the so called 'war on terror' just as both Mr Cameron and President Obama were hoping to benefit from the dividends of a speeded up withdrawal from Afghanistan.
And one other point. While we back war on al-Qaeda-linked militants in Mali, we support those fighting alongside them in Syria.
For Cameron and Obama these are difficult, tangled issues.