The president decided to change his plans because he wanted the war to be over with, If the us was to invade ALL of japan then it would’ve taken so many more years because of the constant fighting and the is didn’t want to take on more casualties than they already had. Thus, he chose two cities Hiroshima and Nagasaki as his choices. Considering they were the two largest cities. When they had dropped it there were so many civilian and possibly military casualties that Japan decided to surrender to America because they possibly thought that the USA had more nuclear weapons. Therefore ending the war with the axis powers.