Won't happen, at least not in any meaningful form.
Baltics or Poland are existentially threatened by Russia, Spain or even Germany are not, even if Russia can do a limited damage to them. What is supposed to create "unity" in that regard? What would force Spain to contribute as much as, let's say, Finland? We can see even now, with all these US threats, not every NATO country was willing to increase its spending on military. And even more importantly, who is going to command such EU army? Commission?