As US Secretary of State Colin Powell flies to Ankara, relations between the United States and Turkey are at a low point, with fears over a longer campaign because of the failure to agree a northern front.
In December, when the war on Iraq was just in the planning stage, the United States appeared to take Turkish support for granted.
US Deputy Defence Secretary Paul Wolfowitz, after flying to Ankara for talks with Turkish leaders, said that "Turkish support is assured.
Turkey has been massing troops on its eastern border
"Turkey has been with us always in the past, and they will be with us now," he said at the time.
The US was planning to move tens of thousands of troops, including the crack 4th Infantry Division, through Turkey to open a northern front in the war.
In the event, the Turkish Parliament rejected the deployment by three votes on 1 March, despite the presence of dozens of ships carrying US equipment waiting offshore to unload.
Last week Mr Wolfowitz admitted that "if we had an armoured force in northern Iraq right now, the end would be closer".
Politically, the failure to gain Turkey's full co-operation has also weakened the US claim to have the support of moderate Muslim states for their actions in Iraq.
No common ground
The US had underestimated the growing popular opposition to the war in Turkey, and the political inexperience of the new government, then led by Abdullah Gul.
And some observers believe the Turks over-estimated their importance to the war effort, which they thought they could delay by inaction.
Turkey had been a strong supporter of the 1991 Gulf War, opening up Turkish airspace and offering Turkish troops in Kuwait.
But Turkey feared that the creation of a post-war semi-autonomous Kurdish state in northern Iraq would encourage separatist zeal among its own Kurdish militants, whom it was already battling.
And Turkey has suffered economically, entering a major economic recession that started in 2000.
The economic crisis was very much on the mind of Turkish negotiators when they came to Washington in February.
The then Turkish Foreign Minister Yasir Yakis reportedly demanded $92bn over four years in aid in return for Turkey's support in the crisis - 20 times the $4bn the Bush administration was said to have offered.
The Turks wants to stop Iraqi Kurds moving towards independence
The Turks also demanded written guarantees that Congress would pass any aid deal agreed with the president.
Eventually, after late-night negotiations with Mr Powell, the Turks won a larger package - reportedly $6bn in aid plus loans - and President Bush told the Turks they were "great negotiators".
But the public spectacle in the US press, which accused them of "haggling like carpet salesman", embarrassed Turkey.
At the same time, French President Jacques Chirac praised the Turkish position, and encouraged Turkish leaders to prevent the war by meeting Arab foreign ministers and hosting a peace conference - and perhaps threatening difficulty with Turkey's hope of EU admission if it appeared too pro-war.
Polls showed that 90% of Turks opposed the war.
No high-level US officials visited Turkey, and few seemed aware of the growing political difficulties - and ambivalence - of the Turkish prime minister.
Instead, US Vice-President Dick Cheney called Mr Gul and asked for an early vote on deployment - to take place during the Muslim holiday of Bayram - and it was rejected.
Perhaps the biggest miscalculation by the US was the idea that they could rely on the Turkish military to ensure that their request was honoured.
They failed to see that the military had less influence on the new, more radical Gul government.
And they failed to take into account the real worries in the military about the threat of fresh Kurdish unrest.
Now the Turks are considering whether to move more of their own troops into northern Iraq, bolstering the force already there to combat Kurdish militants.
Mr Powell is flying to Turkey to try to dissuade the Turks from going ahead with the deployment.
His fence-mending mission comes at a time when Turkey is more alienated from the US than ever.
Ankara has, until now, proved far more resistant to American pressure than anyone had thought.