So far, we have been talking about the offensive cyber capabilities of some of the most important cyber powers (including the U.S.). But what about cyber defense? How do you defend against cyber-espionage, cyber-sabotage, or disinformation? Is there an effective way of defending against each, either in or outside of cyber space? How do you determine a proportionate response to an attack on a dam or a power grid? How do you determine who should respond to a cyber-attack on a corporation (the corporation attacked, the military, or the intelligence agency)? How do you avoid escalation that could lead potentially to the use of weapons of mass destruction (chemical, biological, or nuclear)? What could restrain countries from over-reacting, that is, from using disproportionate retaliation in responding to a cyber attack?