(June 5) On its final descent into Istanbul yesterday, my plane swooped low over the length of the Bosphorus, that narrow sluiceway between continents, before landing at Ataturk International Airport. It was a thrilling and weirdly modern way to enter this ancient city, retracing the invasion route which the Persians used to defeat the Scythians two millennia ago, and which the Turks used to dislodge the Byzantines in 1453, and which millions of tourists follow today.

Invasion routes are not entirely the stuff of history. For it was the Bush administration's failure to persuade Turkey to let the U.S. invade Iraq from the north that may rank as the single biggest diplomatic blunder of a presidency for which that is quite a superlative.

So much stemmed from that single and quite avoidable failure. Americans today barely remember it -- one of many mildly depressing memories from the bellicose winter of 2002-2003. But its effect was enormous then and now. Surely the insurgency would have been weakened and perhaps decapitated by US forces coming in both directions, and the desperate remnants of Saddam's regime would have had none of the leisure they enjoyed to plan the post-war reception they have been giving American personnel ever since.

At the end of the Clinton administration, US-Turkish relations were excellent, thanks in part to a successful trip President Clinton made in 1999 (I participated as a speechwriter). Under President Bush, the relationship declined for numerous reasons (polls have measured a precipitous drop in pro-American feelings since 2000). The nadir was plumbed in March 2003, when the Turkish parliament refused to allow the US to invade Iraq. Neither side behaved perfectly at the time, but surely the burden for a persuasive argument was on the Americans. If a NATO ally living next door to Iraq was not sufficiently alarmed by Saddam's threats to go to war, then there was something seriously wrong with our argument. It was not just the lack of hard evidence -- it was the dismissive way we demanded the right to invade without high-level consultation. Friends don't treat each other like that.

In retrospect, this was as big a strategic error as the decision to send too few troops. In the past, many Republican presidents offered great leadership on Turkey -- Eisenhower and Reagan spring to mind. And the relationship is vital to Turkey, as anyone can tell by looking at other military episodes -- in 1950, when the Turks were hugely important allies fighting Communist aggression in Korea, and in 1991, when they formed a smaller but still meaningful part of the first Gulf War alliance. Since then the relationship has improved, thanks to efforts on both sides. But in 2003, when we really needed delicacy guiding one of our most important relationships, it was nowhere to be found.

Going to war is not just about lining up the right hardware and the right battle plans. It's about bringing the right partners on board. That single blunder continues to haunt the valiant Americans trapped in Iraq, whether we remember it or not.

Ted Widmer is a historian who has taught at Washington College and is moving this summer to Brown University, where he will direct the John Carter Brown Library. From 1997 to 2001 he was a foreign policy speechwriter for President Clinton.