The refugee/illegal immigrant/asylum seeker, and yes a minority of terrorists likely hidden among them are all a problem of the here and now. It bears no relationship to the events of 80 odd years ago the causes are more complex and the migrants fit no single pigeonhole.
It is more useful to look at the causes of the migration rather than put labels on those moving.
1)Regime change, civil war,persecution, genocide
2)Climate change. Semi desert areas such as the Sahel have always been vulnerable to slight shifts in climate. Plant growth is already challenged and has little resilience. Global warming, constant war, political instability, burgeoning population only leads to unhappy outcomes.
3)population growth
4)Western political meddling, raw material extraction.
5)Economic migrants this also includes millions from Eastern Europe moving entirely legally to the west in search of a better life.


You may like to separate terrorism from the refugee problem but as in many cases they originate from the same countries they are both intricately linked. We also have european jihadis further complicating and compounding the problem. The presence of potential terrorists among any group of immigrants inevitably requires higher levels of security to deal with them. You may not like the idea, or even refuse to accept the reality, but the alternatives are potentially far more damaging
To demonstrate this point let us consider Syria. It is fighting a war for it’s survival. The Syrian defence forces are fighting a rag tag and bobtail soup of insurgents, many aided and abetted illegally by western forces. This has resulted in 2016, from an estimated pre-war population of 22 million, the United Nations (UN) identified 13.5 million Syrians requiring humanitarian assistance, of which more than 6 million are internally displaced within Syria, and around 5 million are refugees outside of Syria.

Another illustration is that of France in the
sahel and the increasing presence of US bases in the region.

Neatly forgotten is that Syria was in the grip of reputedly the worst
drought in 900 years before the insurrection broke out The drought caused 75 percent of Syria's farms to fail and 85 percent of livestock to die between 2006 and 2011, according to the United Nations. The collapse in crop yields forced as many as 1.5 million Syrians to migrate to urban centers, like Homs and Damascus.
The drought had displaced Syrians long before the conflict began," said Francesco Femia, president of the Center for Climate Security. "And what is frightening is that analysts who study the region completely missed it."

As a total aside: In this way the Syrian civil war and the hundreds of thousands of displaced, who are seeking refuge in Turkey and Europe, could be seen as a foreshadowing of a much more alarming humanitarian situation should nation's fail to keep global temperature rise under control.
The Pentagon has long identified climate change as a "threat magnifier," a factor that can aggravate already existing political fault lines. And the G7 issued a report in June warning that climate change "will aggravate already fragile situations and may contribute to social upheaval and even violent conflict."
An interesting article in blacklisted news further illustrates the complexity of the situation
https://www.blacklistednews.com/article/66685/ben-rhodes-admits-obama-armed-jihadists-in-syria-in-bombshell.html
Of who did what to whom and an even more interesting article from Russia today illustrates the fallout in Europe
https://www.rt.com/op-ed/430820-eu-migration-crisis-merkel/

I am afraid one paragraph submissions on the subject of refugees cannot even begin to scratch the surface. It is complex in terms of origin, outcomes and potential solutions. Of course there are those will look at the source of the links and make no attempt to verify the content, content in their smug satisfaction that they already know the reasons.