Occupying ISIS Land
Obama has very good reasons not to like the idea of an occupation. The Iraq and Afghanistan occupation have been disasters. At least part of the reason is that those occupations were monumentally stupid. By contrast, the occupation of Japan and Germany after WW II were big successes. Bush's failure to plan for an occupation was one of his biggest blunders. The US started planning for the occupation of Germany and Japan right after Pearl Harbor.
Of course Bush also made many other specific blunders, like disbanding the Iraqi army but allowing them to keep their weapons, producing a vast band the armed and unemployed. If Obama, or some future President, does decide to destroy ISIS, he needs to start planning now, and study the lessons of World War II and the American Civil War.