Nuclear weapons and global warming stand out as two principal threats to the survival of humanity. In each of these existential cases, two strategies born during the Cold War years are competing: abandon the respective systems, or defend against the consequences, once the harmful effects produced by those systems occur. The first approach to the nuclear and climate threat focuses on arms control, non-proliferation and disarmament and on greenhouse gas emission reductions and mitigation. The second approach involves active defense: in the nuclear realm, missile defenses against nuclear-armed delivery systems; for climate change, geoengineering that removes carbon dioxide from the atmosphere or changes the Earth’s radiation balance. The more policies fail to reduce and constrain the underlying drivers of the nuclear and climate threats, the more measures to defend against the physical effects may seem justified. Ultimately, the overarching policy question centers on whether nuclear war and catastrophic climate change can be dealt with solely through reductions in the drivers of those threats, or if active defenses against them will be requested. This article is free-access through September 30, 2019.