Also, what's offense and what's defense? Anti-aircraft artillery (effective against drones? I think current air drones are optimized for use against low-tech enemies w/ few defenses) is a "defense" against 'attack from the air', but 'heat-seeking AA missles', 'flack guns', 'radar-guided AA missiles' and 'machine gun turrets' are all "offenses" against combat aircraft where the defenses are evasive maneuvers, altitude, armor, and chaff/flare decoys.

In WWI, defenses (machine guns and fortifications) were near-invincible, and killed attackers without time for them to retreat.

I think that current drones are pretty soft and might even be subject to hacking (seem to remember somethign about unencrypted video?) but that would change as soon as somebody starts making real countermeasures.