In his titanic efforts to stem decline and reform America, Obama has shown vision and talent, but he also learned the hard way that, as Henry Kissinger put it in his memoirs, the pledges of new administrations are almost invariably like “leaves on a turbulent sea.” The impossible deadlines, the always ambiguous information the president is fed, and the complex choices that he must make are too frequently bound to clash with political constraints and the resistance to change of both allies and foes.