Hiroshima

With the number of survivors rapidly declining and their average age now exceeding 86, this year's anniversary is considered the last milestone event for many of them.

Japan marks 80 years since the U.S.'s World War II nuclear attack on Hiroshima. The number of atomic bomb survivors is dwindling, as wars and global instability pose a growing challenge to Japan's pacifists.

President Obama will be the first sitting U.S. president to visit the Japanese city since America dropped an atomic bomb on Hiroshima and Nagasaki at the end of 1945.