World leaders invariably refer to their summits as "historic" events, and they often exaggerate. However, there is no question that the North Atlantic Treaty Organisation summit that begins today in the Spanish capital of Madrid fully deserves this label.

For the heads of state and governments of the 30 member-states of this US-led military alliance are grappling with their gravest security crisis in more than half a century. And they are conscious that what they will decide in Madrid over the next two days may shape Europe's and perhaps global security for decades.