It's not an identity crisis. It's not an existential crisis. Life after the Cold War has been remarkably consistent and coherent for NATO -- consolidate Europe west of the historical post-Soviet space and, after 9/11, to share the burden in Afghanistan. But NATO is headed nonetheless for a personality crisis. How are these missions to be carried out? With what disposition? Begrudgingly? Adversarially? Blithely? Cautiously?
The problem is compounded now by Turkey -- and by frustrated US reactions to the Turks. There is only one reason why Turkey is in NATO, and that is because Turkey sealed off the Mediterranean to the Russians. A little pressure in Europe, and the alliance will not crack. A little pressure in Turkey, and people start asking questions.
NATO can carry on just fine without Turkey. But why should it? Do we really think that the same Russians who planted a flag on the North Pole wouldn't turn their eyes back to the Bosporus if NATO stopped at Greece's edge? Do we imagine that Russian-Ukrainian relations wouldn't immediately worsen over the already contentious issue of the Crimea and the Black Sea Fleet?
I'm not a Russia scaremonger. I don't think Russia is our enemy. I don't think Georgia would belong in NATO for quite a while, and I don't think Ukraine would belong in NATO until Russia itself does. But I do think Russia would gladly trade all the Baltic NATO exercises in the world for an outlet onto the Mediterranean (and, thence, points southeast), and that's simply not in the interest of NATO or the US. So for all those grumbling about kicking Turkey out of NATO, a word of caution: a personality adjustment is in order.