It was 1945, the Italians had surrendered, the Germans had surrendered, Japan had been under heavy under attack with conventional bombs (leaving tens of thousands dead and most of their cities destroyed). USSR was also there (had freed Manchuria). Besides the official US line, held for political reasons, most countries in the world believe this was the case, i.e. the US was just itching to test their new weapons in a war setting.

"In his 1965 study, Atomic Diplomacy: Hiroshima and Potsdam (pp. 107, 108), historian Gar Alperovitz writes:

(...) In mid-April [1945] the [US] Joint Intelligence Committee reported that Japanese leaders were looking for a way to modify the surrender terms to end the war. The State Department was convinced the Emperor was actively seeking a way to stop the fighting."

I've read similar arguments and evidence in my language books, but here's some more from a quick search on the web (the source seems dubious in general, but the specific parts legit):

"Months before the end of the war, Japan's leaders recognized that defeat was inevitable. In April 1945 a new government headed by Kantaro Suzuki took office with the mission of ending the war. When Germany capitulated in early May, the Japanese understood that the British and Americans would now direct the full fury of their awesome military power exclusively against them."

and:

"It was only after the war that the American public learned about Japan's efforts to bring the conflict to an end. Chicago Tribune reporter Walter Trohan, for example, was obliged by wartime censorship to withhold for seven months one of the most important stories of the war.

In an article that finally appeared August 19, 1945, on the front pages of the Chicago Tribune and the Washington Times-Herald, Trohan revealed that on January 20, 1945, two days prior to his departure for the Yalta meeting with Stalin and Churchill, President Roosevelt received a 40-page memorandum from General Douglas MacArthur outlining five separate surrender overtures from high-level Japanese officials. (The complete text of Trohan's article is in the Winter 1985-86 Journal, pp. 508-512.)

This memo showed that the Japanese were offering surrender terms virtually identical to the ones ultimately accepted by the Americans at the formal surrender ceremony on September 2 -- that is, complete surrender of everything but the person of the Emperor."