We never learn
President Barack Obama recently said that his failure to follow through on the toppling of Libya's former leader Muammar Gaddafi, which occurred in 2011, and failure to plan for the aftermath of that event -- which US and NATO airstrikes helped bring about -- was the worst mistake of his seven years in office.
Libya has descended into anarchy, and Isis has now established a presence of an estimated 5,000 fighters within striking distance of Europe.
It is hard to imagine what could be meant by "following through" other than military occupation. The mistake was not the failure to follow through, it was destabilizing the region in the first place.
The region might have not been totally stable under Gaddafi, but we keep making things worse by becoming involved in these conflicts - repeating the same mistake over and over again.