There is an appropriate time to take the offense. If you are dealing with multiple assailants, then it would be much wiser to make the first move instead of waiting for the attack to come to you. You will have to disguise your true attack with a feint.
I have only recently begun to see the true aspects of the internal (neija) requirements. There is a saying, and it goes something like this: on first notice of your opponent attacking, one reacts and makes the first strike. Similar to the Bruce Lee quote mentioned previously.
Fortunately, the taiji system was designed to defend and attack at the same time. One hand is defending the primary assault leading the opponent to a less favorable position and the other hand is attacking.
I'm saying hand to keep it simple, but in all honesty, we know that it's about the entire body reacting properly. Emptying one side to neutralize and absorb while the other is transferring energy to the full side.
When opening/closing the door that is being attacked, we are closing/opening on the opposite side to discharge. I use opening/closing and closing/opening because it will depend on the situation.
If we take this view, then you can see that attacking contains defending components and vice versa. I believe that is the true science we are learning to become proficient in from a martial arts perspective.

Trying to do no harm while trying not to get harmed seems a lot more appealing, and certainly healthier.