Under nuclear deterrence, the development of nuclear weapons causes an aggressor state to not challenge a nuclear-armed defender, even though they would have challenged in the absence of those weapons. This lecture explains why there is a consensus that nuclear weapons are useful for deterrence, despite debate on whether they are useful for compellence. We also learn about the Blue Peacock, a nuclear weapon powered by chickens. Seriously. Photo of the White House is licensed under CC BY-SA 3.0 Unported: https://creativecommons.org/licenses/by-sa/3.0/deed.en