That would be a start.

A lesser act would be for any European nation to arrest the operators of the invasion craft and the NGO officers under smuggling charges. Also confiscate all of the vessels used in the smuggling and invasion operation. Sell them. or scrap them. or put some to use as military intervention craft to protect the sea routes against further invasion.
Send all of the invaders back.
Try those who set up the invasion by allegedly “good” acts meant to bring the invasion.
Example: Starting the wars in Syria and in Libya and all of what occurred in North Africa and the Middle East.

There is more. I think you can get the gist of what must be done.

These acts to change Europe will result in much more blood in which those who set it up must be tried in war crimes or crimes against humanity trials, or over a drum head, given what future conditions prevail.