BY PICKING Chuck Hagel to be his defence secretary and John Brennan to succeed David Petraeus as director of the Central Intelligence Agency, Barack Obama has made plain the qualities he is looking for in his most senior security officials: experience, caution and, perhaps above all, personal loyalty. Both men see the world very much as Mr Obama does, which is to say, complicated, messy and all too frequently impervious to the use of American power even when wielded with the best of intentions.