The decisions to incinerate Japanese cities were not made within days of the Pearl Harbor attack. They were made after 3 years of bloody, savage warfare and in the context of unsupportable (to Americans) losses on the battlefields in the Central and SW Pacific areas of operation. In the aftermath of Pearl Harbor, our war aims were to sweep the seas of the IJN and to thereby compel surrender to our long-term goals in East Asia.
I do not believe that Roosevelt would have ordered the incineration of Hiroshima and Nagasaki on December 8, 1941, and I bet you don't, either.
I didn’t write anything about Hiroshima and Nagasaki. But surely you have heard of the Doolitle raid, the 75th anniversary of which was celebrated last week. There was certainly civilian “collateral damage” from that attack on Tokyo, which was slightly more than four months after Pearl Harbor. From an article on historynet.com:
http://www.historynet.com/aftermath-doolittle-raid-reexamined.htm
“Even as crews were recovering American dead from Pearl Harbor’s oily waters, President Franklin D. Roosevelt was demanding that his senior military leaders take the fight to Tokyo. As Army Air Forces chief Lieutenant General Henry Arnold later wrote, ‘The president was insistent that we find ways and means of carrying home to Japan proper, in the form of a bombing raid, the real meaning of war.’”
That raid provoked the Japanese to attack Midway, and we know how that turned out for them.