What Empowers ISIS

  Whether or not ISIS brought down Metrojet Flight 9268 in the Sinai Peninsula, the terrorist group has catapulted itself from obscurity to lead the global jihadist movement in just 18 months.

ISIS controls territory in Syria and Iraq that by some estimates is the size of the United Kingdom, and it lords over millions of people in both countries.

The group has also secured pledges of allegiance from two dozen militant organizations from around the Muslim world, including in the Sinai and Egypt's neighbor Libya, while around 10 other groups have declared some form of solidarity with ISIS.

The key to ISIS' success is not the group's military strength -- ISIS in Syria and Iraq may number only about 20,000 to 30,000 fighters -- but the weaknesses of the regimes where the group is doing well.

Think of the Sunni militant group ISIS as a pathogen that preys on weak hosts in the Muslim world.