"..The most underestimated cause (per the question prompt) is very likely the occupation of Islamic lands by foreign powers..."

Greg Djerejian in the Washington Note, here

"...Last, I would say the most underestimated cause (per the question prompt) is very likely the occupation of Islamic lands by foreign powers. This has historically been a major cause of Palestinian terrorism (see, over the years, the PLO, Hamas, Islamic Jihad, PFLP, DFLP etc), and see too Chechnya, Lebanon (Hezbollah notably), and more. .."