I'm going to defer to George S. Patton. Offense, all the way. Attack with overwhelming force leave nothing of your enemy but smoking ruin and the need for a defense if moot. Besides, it much harder to protect than destroy.
Witness the hawkish behavior of the US of late. Sure it's at least partly about re-election in 2004, but it also at least partly based on the premis that the most effective defense is to "vanquish your enemies and chase them before you, to rob them of their wealth and see those dear to them bathed in tears, to ride their horses and clasp to your bosom their wives and daughters." Or maybe that was the greatest pleasure, I can never keep that straight. Say what you will about defense, but history has shown it a poor substitue for offense.