You could argue that world war 1 the machine gun meant that defense was higher than offense, of course then the tank was invented which combined with infiltration tactics helped offense regain some of it's capabilities. You could also argue that the later parts of the us civil war defense was stronger than offense. The battle of cold harbor is a pretty good point in this direction as perhaps is the battle of the crater though that one can be blamed on poor coordination. However coordination is one of the hardest things for offense to get right especially pre radio.