If terrorism is your one fear that's one thing.
If instead you believe the only conceivable reason to accept migrants is 'white guilt' or 'sense of obligation' then that's something else entirely. Since the points i'm arguing against seem to be in favour of the latter this is the line i'm pursuing.
Obviously this is an issue of separation of religion and state, considering Islam is the fundamental core of every political structure in the Middle-East. It is also a huge burden on the national security of every country with a sizable Muslim population. Islam has no central authority structure or legitimate figurehead to account for it's ideology when used as a political weapon, which is quite frequently. This makes it's compatibility with western society concerning and questionable.
In a conflict that is multi-factional and multi-denominational such as Syria these concerns become increasingly alarming. ISIS holds territory with an estimated population of 10+ million, and for years has also held control over the major border crossings with Turkey. They also force the local populace in territories they control to participate in religious education camps, where thousands of people including young children are brainwashed, taught how to execute and torture prisoners, operate firearms, and build improvised explosives.
Politicians have known about the refugee crisis for years. It's just now that it's become a problem they can no longer ignore because inaction threatens the integrity of the EU. Some nations, such as Sweden, strive to become a future Islamic caliphate and utopia and are blindly leaving their borders wide open with no concern for their own citizens. The US harbors 12+ million undocumented economic migrants so I doubt 1 million with break the bank or change much of anything for the majority of EU nations, but I still don't think it's worth putting your own national citizens security as collateral when all these refugees could just be taken care of closer to to the Middle-East like they have been for the last 5 years.