Offense is action, defense is mostly reaction.
Offense sends out personnel. Defense reacts to that.
Offense sets a formation. Defense reacts.
Offense runs or passes. Defense reacts.
Most of the time if a offense is reacting to a defense is because that defense has superior talent at a certain position, like a defensive end who can't be blocked one on one or a corner back who you can't throw to his side of the field.