A preemptive action is made to keep some other action from being taken. Before you could be accused of eating the whole cake, you decided to make a preemptive apology, which was met with awkward silence.
Tired of being disturbed by your roommate's loud music, your preemptive strike was to crank up your favorite opera as soon as you arrived home. Unfortunately, he'd already made a preemptive strike of his own by borrowing your stereo. Don't confuse preemptive with peremptory, a word meaning “arrogant.” You don't have to be arrogant to make a preemptive move, just cautious.