For decades Turkey has been a generally reliable partner in an unstable region where the West has pressing need of friends. In all questions of grand strategy, Turkey’s interests and those of Europe and America coincide, not least in securing a more stable Middle East where Islamist extremism has no purchase. Turkey’s successful recent pursuit of free-market reforms, richly rewarded by inflows of foreign investment, should have cemented the relationship and advanced its longstanding ambitions to join the EU. The strengthening of civil democracy in Turkey, and the nurturing of pro-Western sympathies within the country, were expected by Europeans and Americans to go hand in hand.

Instead, events within and outside Turkey have conspired to strain the relationship, to the point where Ankara’s pro-Western alignment can no longer be taken for granted.

Turkey is in the throes of political and social transformation. The solid renewed mandate given this summer to the AKP, a party with strong Islamist roots, has brought to the fore fundamental questions about the extent of commitment to the secularism espoused by modern Turkey’s founder, Kemal Atatürk. Europeans have suppressed their unease, but are no longer so sure that Turkey will prove a model for the coexistence of Muslim values and secular institutions.

The Turks, in turn, feel betrayed by the EU, which they consider with some justification to be double-crossing them in the negotiations over Turkey’s application to join — an application that most Turks now believe the Europeans will never accept. With the US, careful diplomacy on both sides had begun to mend fences broken by Turkey’s refusal to allow its territory to be used in the invasion of Iraq. But much of that quiet work has been undone by an appallingly timed congressional push, dictated more by Democratic concern to garner support from rich Armenian-Americans than by the demands of historical justice, to declare the Turkish empire’s mass deportation and slaughter of Armenians in 1915 an act of genocide. Much unnecessary damage has been done by the inept Nancy Pelosi, the Speaker of the House of Representatives.

How much damage will soon be put to the test. Turkey is poised to attack the bases established in northern Iraq by PKK guerrillas fighting for autonomy for Turkey’s Kurdish population. Yesterday, the PKK killed more than 17 Turkish soldiers, putting the Government under more pressure to act. A full-blown invasion, by the 60,000 troops massed on the border, would be disastrous for the one stable part of Iraq. But, as Turkey’s Prime Minister, Recep Tayyip Erdogan, makes plain in his interview with The Times, the Turks are in no mood to listen to anguished Western pleas for restraint.

Britain’s strong support for Turkey’s EU membership will give Gordon Brown leverage in the meeting scheduled with Mr Erdogan in London this week; but even so he will need to choose his words carefully. He should play for time, pointing out that Turkey is set to host an important regional conference on Iraq. He could suggest, quietly, that an invasion could revive waning PKK fortunes in southeastern Turkey. But he should also listen, and Kurdish officials must take Turkey’s concerns more seriously.

Turkey is in intransigent mood, but that is in part because it feels deserted by those who should be its friends. Evidence of friendship during difficult times is a useful message for Mr Erdogan to take back home.