Support

• The Pacific War
  The final battles of the Pacific War, most famously the Battle of Okinawa and the Battle of Iwo Jima, resulted in horrific casualties on both sides. Allied leaders didn't want to risk a repeat of those kinds of massive death tolls that an invasion of Japan was expected to cause.
  

• Kamikaze and bushido:
  The actions of the kamikaze demonstrated the lengths that the Japanese would go before they surrendered. In addition, the radical military leaders, armed with the code of bushido, mandated that every citizen, soldiers and civilians alike, die fighting the enemy. This lead the Allied leaders to believe that an invasion would require mass casualties on both sides.
  

• The Manhattan Project
  The U.S. government needed to justify the $2 billion invested into the Manhattan Project to the U.S. public. WIth the kind of resources that were invested, they were under pressure not to "waste" one of the three atomic bombs on a "demonstration."
  

• Horrific casualties by invasion
  An invasion would not only have resulted in horrific casualties of Allied soldiers (according to the U.S. Army Chief of Staff, around 500,000) but also mass casualties on Japanese civilans. This estimated toll was far greater than the deaths caused by the atomic bombings.
  

Opposition

• Pearl Harbor
  The attacks on Pearl Harbor inflamed U.S. sentiments against Japan almost bordering on near genocidal. The decision to drop the bombs may have been influenced by racism and exacting revenge for Pearl Harbor.
  
  
• Scientists of The Manhattan Project
  In the days before the bomb was dropped, an overwhelming majority of the scientists of the Manhattan Project wrote and signed a petition against the use of the bomb. They sent this to the President who never received it. Many of them also aruged that the power of the bomb could be conveyed without killing civilians.
  
  
• Overblown casualties
  While General Marshall, the U.S. Army Chief of Staff, estimated that 500,000 people would die in an invasion of Japan, another report filed by U.S. Joint War Committee projected a much lower number. Marshall's estimate was used to help justify the bombings to the public.
  
  
• Civilian Targets
  The two target cities had little "military value" and were chosen more to measure the effects of the bomb. In Hiroshima alone, civilians outnumbered the troops five or six to one (Dietrich).