Was Europe not united against aggression before the EU? That has to be the lamest argument ever. One doesn’t have to have a remote entity in Brussels setting local policy to be united against a threat.
The current aggression is the invasion of military aged Middle Eastern males made possible by the EU policy of open borders. In order to stop the current aggression the only way to do it is to secure the border, therefore, the way to stop aggression is to leave the EU, and its ruinous policies, behind.