LONDON • Nato summits are fairly predictable affairs. With 29 member-states, the US-led military alliance in Europe tends to treat summits as periodic reaffirmations of collective marriage vows, carefully scripted events at which every single detail is predetermined, and all that Nato leaders need to do is fly in, smile throughout, and then take up their pre-arranged places for the "family photograph" that concludes the proceedings.

But matters cannot be more different with the Nato summit taking place this week in Brussels. For this gathering not only threatens to be one of the most unpredictable in the alliance's history, but also, most extraordinarily, the source of unpredictability is now Nato's fundamental pillar: the United States, whose leader appears to question the very purpose of this military pact. It is no exaggeration to say that what will unfold in Brussels over the coming days could define relations across the Atlantic between Europe and North America for decades to come, and represent just as fateful a historic departure as the end of the Cold War in Europe more than a quarter of a century ago.