Enemy forces will respond to your tactics. For example, if you perform a lot of headshots, enemy soldiers will soon be supplied with helmets.
Fulton response (shooting down Fulton balloons)
Headshot response (supplying helmets)
Covert actions response (deploying decoys, directional mines, and surveillance cameras)
Combat response (supplying shotguns, body armor, and other equipment)
Nighttime response (supplying flashlights and night-vision goggles)
Sniper response (deploying sharpshooters)