'Tell me how this ends' - the rhetorical question repeatedly posed by Lieutenant General David Petraeus as he and his troops marched on Baghdad in 2003. Four years on, the American soldier has got to come up with some answers. Lt Gen Petraeus – described as a unique combination of warrior and intellectual - has just been appointed the US military’s most senior man in Iraq. Robert Fox assesses why Bush has picked him to do what’s been described as one of the toughest assignments since Vietnam.