The National Security Archive has released an update collection of declassified documents about the decision to use atomic bombs on Japan and the results of the attacks here.
From the beginning, the claims that the atomic bombings averted an invasion that would have killed many (even 1 million, in some versions) Americans and were the only way to end the way quickly have been hotly contested. Dwight Eisenhower (later President), Generals Marshall and MacArthur, and many others descried their use as unnecessary and unethical. Later historians pointed out that Japan had already offered surrender asking only to preserve the Emperor (which the U.S. refused but then allowed after surrender) and that the timing had more to do with Soviet entry into the war than with an invasion.
You can reach your own conclusions after studying the primary sources here.
