A credible European deterrent – one that could prevent, for instance, a rapid Russian incursion in the Baltics – would need a minimum of 1,400 tanks, 2,000 infantry fighting vehicles, and 700 artillery pieces. This is more combat power than currently exists in the combined French, German, Italian, and British land forces.
Why tanks and not air defense though?
Aerospace reconnaissance and ground-air-missiles are much more important to catch ballistic missiles and other attacks than tanks, which are slow and can be steadily bombed over time.