Pros: 1. America was able to secure Japan's final surrender (World War II) without incurring casualties from directly invading a highly industrialized nation that had just over half its population.
2. It was also able to make a show of power to the Soviet Union (Communist Russia), which was the other major power and was in the process of conquering Germany from the east.
3. Building on that, Japan surrendered faster out of fear than it would have if a regular invasion had taken place. President Franklin D. Roosevelt had gotten the Soviet Union to agree to help the U.S. fight Japan, before the U.S. started to suspect the Union's expansionist motivation. Harry S. Truman became president after FDR died, and was rightly suspicious of the Soviets. As a result of the bombing, Japan was never invaded by the Soviet Union. In context, the Soviet Union invaded the northern half of the then-Japanese province of Korea, while the U.S. took the southern part. That's how North and South Korea were formed, and how the southern one was democratic while the northern was communist. Hence, the Korean War, five years later.
Cons: 1. 200,000 Japanese, mostly civilians, died from radiation and injuries caused by dropping the bombs on Hiroshima and Nagasaki. The bombs could have been dropped on an unpopulated area, which would have scared Japan without killing so many.
2. Since the bombing scared a nation into surrendering, it was technically terrorism on an international scale.
3. It was the start of the Cold War. Then, only the U.S. had nuclear weapons. Today, around a dozen countries do. Now, the U.S. wasn't the only one doing research, but the unveiling encouraged others to work harder to catch up.