CAMBRIDGE, Mass. — President Barack Obama was the world's favored candidate in what was America's first global election. The key question is how the Obama administration will tap this rare good will to re-establish U.S. credibility and repair its reputation. How Obama manages issues in the Muslim world will determine the success or failure of his foreign policy because it is here that the greatest challenges lie, especially in dealing with the two U.S. war fronts in Iraq and Afghanistan.