Complex sentence: Theoretically, if he enters a blank or incorrect countersign twice, he will be shot down; however, by that time, the pilot is usually so close to base that he can land before friendly air defenses have a chance to respond.
Split into simpler sentences: Theoretically, if he enters a blank or incorrect countersign twice, he will be shot down. However, by that time, the pilot is usually so close to base that he can land before friendly air defenses have a chance to respond.