By the time George W Bush left the White House perceptions of the United States in the wider world were overwhelmingly negative. As the Obama presidency enters its final phase how have attitudes shifted? The promise was clear - a greater focus on soft rather than hard power and a "yes we can" commitment to healing political wounds at home and abroad. Stephen Sackur speaks to the US ambassador to the UK, Matthew Barzun, a close Obama ally. Has the promise been fulfilled?