When and why did Barack Obama win a Nobel Peace Prize?

In 2009 Obama won the Nobel Peace Prize for bringing together, or attempts to bring together, people with differing aims and outlooks. He took several actions connected with promoting peace where conflict reigned:

banning excessive interrogation techniques; attempted to close the military detention at Guantánamo Bay, proposed a “fresh start” to strained relations with Russia; and reached out to the Muslim world on a trip to Cairo.