The world is agog at the prospect of a Barack Obama presidency, but are expectations of a revolution in US diplomacy justified? No US leader, including Obama if he beats Republican John McCain, can wave a magic wand to bring peace to the Middle East, or halt climate change, or force "rogue states" to renounce their nuclear schemes.  No president can ram through an international treaty or trade agreement if Congress takes umbrage.  What a president Obama would bring, however, is a vast well of goodwill in a world thirsting to re-engage with the sole remaining superpower after eight years of President George W. Bush.  "There is an acknowledgement on both sides of the Atlantic that both the Americans and the Europeans need to manage expectations," said Derek Chollet at Washington's Center for a New American Security.