Americans have not always relished scrutiny of the atomic blast that decimated Hiroshima, Japan, on August 6, 1945. No sitting president has ever visited the city, and in 1996, the United States and China rejected a Japanese proposal that Hiroshima become a World Heritage site. But on Friday, Barack Obama’s motorcade will pull up to the Hiroshima Peace Memorial, and the world will pause for a few moments to contemplate what happened here.It’s one of the most dramatic stages a president could speak from, and obviously compelling to a president who relishes a challenge.