More than four years after the fall of Baghdad, it beggars belief that anyone still has to be persuaded that invading Iraq is a bad idea. Yet Turkey's army is now massed on its south-eastern border, poised to deal what it presumably supposes would be a killer blow to guerrillas of the Kurdistan Workers' party (PKK) based in Iraqi Kurdistan.

Robert Gates, US defence secretary, on Sunday warned Ankara against invading. But Turkey's chief of staff formally sought his government's permission to launch a cross-border offensive last week. There are reasons to believe he will get it - none of them convincing.