If you have a big country defending you for 70 years you get lazy about your own defense, and then when that defense is threatened you panic because you aren't even close to prepared to do it yourself.
Discussion
Maybe it's a demographics thing too, so many young European men lost in WWII they might not have recovered that spirit yet.
I get that they've been reliant on America but that still doesn't answer why they chose to do that in the first place. They used to be so much more proud and actually influential around the world. The British were a legit empire.
good times create weak men and so on ...