Deceive the player
Intelligence isn't always accurate
Have the player briefed for a very tough mission for example. When they arrive, let's say at the ground attack area, everything is already wrecked. A lone enemy recon plane is low, slow, and unarmed. Give them a few minutes to shoot it up. Then that's when the enemy fighter A/C show up. Or alternatively, that's when a whole slew of unarmed enemy transports parade by, and the player has wasted time and ammo on the hapless recon plane, and might miss the most glorious sitting duck target of his career!