Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Aerzen, Germany (CNN) In a cold world, President Barack Obama has found some warmth in Germany. For the famously reserved commander in chief, German Chancellor Angela Merkel has become his closest global partner, an alliance-turned-friendship forged by mutual political interests and parallel personalities. Obama arrived in Hanover, Germany, on Sunday to lend Merkel his backing as she faces political blowback over her stance on refugees fleeing war in Syria, a position Obama praised as a matter of moral fortitude. "She's on the right side of history on this," Obama said alongside his German counterpart Sunday, praising Merkel for confronting some "very tough politics" in opening her country's borders to nearly a million migrants last year.President Barack Obama is in Germany on SundayHe will lend support to German Chancellor Angela Merkel 
 Question:Finally, _'s chief of staff was dispatched to the German capital to resolve the tensions, a rare overseas mediation that was effective in cooling tempers.

Barack Obama