I have a simpler view:
Civilizations fall when they are incapable or refuse to defend themselves.
Incapable because they have destroyed their own economic function.
Refusal when they feel their own civilization is unworthy of defense.
Who does that describe?
There's an even simpler reason: civilizations fall when there's more money to made in decline rather than expansion.
To ascribe what is occurring right now in Europe & the US to malice, lack of will, ignorance, naivete, etc is to miss the real motivation, which is there's $trillions to made playing the middle.
When Hussein reads a script that says disallowing refugees is "un-American", what is handlers really mean is (a) Dems get votes, thus huge kick-backs from influence peddling; and (b) Reps get low IQ workers that will do what they're told for under-market wages.
It's a win-win situation for the uni-party. They couldn't possibly give a sh!t whether the citizenry suffers an aggregate decline in its quality of life.