how effective was flak in WW2? was it a meaningful threat to bombers, ground attack planes and fighters or just a vain response to enemy air superiority? did it cause a large portion of losses?
It was arguably effective as a deterrent in that it forced bombers out of more accurate low altitude attack profiles, making bombing far less damaging. However the real losses to bombers were inflicted by fighters.
It was extremely effective.
The Germans used targeting computers to guide multiple batteries on target as the bombers came into range.
https://www.youtube.com/watch?v=HHKH387_WCw
https://www.youtube.com/watch?v=H8zPNMqVi2E