Did the United States need to use atomic weapons to end World War II?

The Allied victory in World War II came at a high cost to both sides. The Pacific theater was only won after the United States dropped two atomic bombs on Japan (the only time that nuclear weapons have been used in warfare). Was this decision necessary? Would the Japanese have capitulated due to conventional warfare anyway? Did the use of A-bombs hasten the end and actually save lives? Could the same goals have been achieved with just one of the bombs?

Pro

 * The terrorist planes of Japan destroyed the majestic Pearl Harbor of the United States in December 1941. To send the reply of the attack to Japan, the US was to use the atomic weapons.
 * The nuclear attack was almost 4 years after Pearl Harbor, and killed several times more people—mostly civilians.
 * The United States wanted a quick end to the war, which has already killed millions. China experienced the worst, with 20 million mostly civilians losing their lives under the Japanese war machine. The United States knew the Japanese were not going to surrender easily, so there were two main options; either undergo a land invasion, which would kill hundreds of thousands, possibly millions more, cost both countries dearly in terms of money, resources, life, and drag the war for many years, or to send a quick and decisive message to the Japanese government. The deaths in both Hiroshima and Nagasaki are only a drop in the bucket compared to the total death toll in World War 2. It was a necessary sacrifice that would ultimately save millions.

Con

 * The Japanese had already engaged in negotiations with the Soviets to end the war, they just needed more time to finalize the details and wrest control of Japanese society from the generals to the emperor.
 * Did it necessitate the US ceasing hostilities against Japan until negotiations were concluded, if they were to conclude, and possibly against American objectives?
 * During The Fog of War biography of Robert Mcnarma, United States Secretary of defense, described the fire bombing of Japanese cities and the corresponding sized American cities. The number of cities is staggering large. In one night the United States burned to death 100,000 citizens of Tokyo. A much larger death toll then the combined nuclear attacks. The United States had no problem killing large numbers of Japanese. Ethically or physically. Would it be more moral to kill less Japanese and have 10,000s United States troops slaughtered on the beaches? Would you care to explain that at your impeachment hearings Mr. President? There was no question that the USA would use the nuclear bombs. Too much time, energy and effort went into producing them, to not use them. Could the USA have won the war without the nuclear bombs, yes, but how many more Japanese cities would be destroyed? What would 'victory' look like?
 * As formulated, the answer is a clear and unequivocal no. Since, the question asks whether they did need, not whether it was the best course of action. Surely it was possible to end the war without the atomic weapons, at the cost of American lives. A more interesting question and the one usually discussed is whether the United States should have used the atomic weapons the way they did.