HIROSHIMA/WASHINGTON — As U.S. Ambassador to Japan John Roos on Friday became the first U.S. representative to attend the annual ceremony to commemorate the atomic bombing of Hiroshima, the move was fraught with both political gains and risks for U.S. President Barack Obama.

Obama shares with Japan the goal of achieving a world without nuclear weapons. The decision to send Roos, one of his closest friends, to Hiroshima was welcomed by the Japanese government and public, with analysts saying it has even generated momentum for Obama's goal.

Experts said, however, that by sending Roos, the Obama administration runs the risk of a political backlash at home. The envoy's attendance at the ceremony fueled speculation in Japan that the president himself may visit Hiroshima soon, even though such a trip remains a sensitive and difficult issue.