World War III stated about a year ago.
Wave after wave of forced immigration.
Forced?
The flood of refugees is a created thing.

That is the First Strike.
Move large numbers of people into a country, and immediately the social supports in that country get tapped out.
The list is long of the resources that 'disappear' or become scarce when the refugees come.
And this is in the first few weeks of any groups arrival.
Then there is housing, food and jobs for the long-term.
Medical attention is costly, and many of the new arrivals will be in need of doctors.
So, [war-torn region] sends out wave after wave of refugees.
It doesn't cost a lot to do that, BTW. 
Pass laws banning religious practices, ban their traditional music, put restrictions on the work they can do.  Slant the Courts to jail members of the group for nothing.  Make their day-to-day lives intolerable.  Throw in some random murder, and the stage is set.

The down-trodden group will be showing up on the shores of the nearest country that has poor relations with the bad place they are leaving.
So [Syria] gets to purge their country of undesirable peoples, Europe gets burdened by the crush of caring for so many, and then the Second Strike hits.

Second Strike?
These refugees are overwhelmingly of one Faith: Islam.
And Islam is clear on the fate of the non-believer.  You either convert, become a slave, or are killed as a threat to the Faith.  There are no other options.
So what if the Sunni refugees were horribly abused by their Shiite government?
The distinction between a Sunni beheading and a Shiite car-bomb is lost on me.
One is simply more fundamental, the other more accommodating.
But both read the same Koran, both believe the Koran to be the ONLY book that should exist, for everyone, forever.

Strike Two will occur about 2-5 years after the 'problem that started it all' is resolved.
Even if 1 out 100 refugees are radicalized or were 'sleeper agents', that is an Army that has had time to get jobs, become neighbors and friends, know where everything is, and how well it is guarded.

The Third Strike will be when the country that sent out this wave of Soldiers attacks their neighboring countries, forcing an already strained EU/NATO response.  Another War, overseas.
Can't win a war on that many fronts.
Agents blowing up vital services [telecom, power and water disruption], massacres, rioting in the streets, political infighting, police strained to breaking, military powerless to stop diffused attacks, war raging overseas.
A country so attacked cannot hold.
EU?  Been nice knowing you.
NATO?  Good luck!
America, keep your borders tight!