The bomb
The nuclear bomb is at the center of this book, but to qualify what the bomb "means" for the earth is a problem that is comprehensive and painfully complicated. In some ways, the bomb is evidence of the human propensity for chaos and malice. In other ways, the bomb represents a built-in condemnation for wrong political behavior, built into the very potential of reality. This book's title hints at this broad spectrum of symbolic meaning by using the word Doomsday. That word is specifically used to reference apocalypse, which has religious significance as well as political and scientific implication.
The ecological threat
The threat of a bomb is more substantial than the blast and its radiation alone. The threat of nuclear war is that a bomb could tear a hole in the ozone layer that might tip the balance toward global warming. With multiple bombs at once, the atmosphere could be subject to a tipping point, where the climate would pass a point of no return, leading to chain reactions in the climate that could alter or destroy the livability of the earth. By going to war with nuclear weapons, the humans risk destroying their own chance of survival, and the planet.
Bureaucracy and President Eisenhower
Clearly, the book should (and does) address the actual threat of warfare. It isn't every person that is capable of launching nuclear weapons. That ability is reserved for politicians and government officials. Ellsberg comments that government is not historically prone to ethical or fool-proof behavior. Sometimes, the problems of human nature are demonstrated even at the top of the chain of command. Ellsberg mentions a dubious decision where Eisenhower gives the power to launch nuclear bombs to his subordinates without his knowledge or permission.
Paranoia and conflict
As the Cold War demonstrated painfully clear, there is a dynamic paradox in geopolitical balance. When two countries are threatening each other with nuclear war, it is unclear to see how the balance can be maintained. There is a feedback loop. On the one hand, paranoia makes conflict more and more likely, but on the other hand, paranoia is the result of conflict potential. When paranoia and conflict converge, Ellsberg comments that nuclear war becomes more and more likely.
Civilian casualties
A central idea for the horror of nuclear war is offered in this consideration: Typical warfare is often done in remote locations, or for strategic advantages. But nuclear warfare is not an attempt to strategically obtain victory through strategic military operations. Nuclear warfare is an attempt to decimate a nation's population by targeting their innocent civilians with the world's most dangerous weaponry. That dynamic is horrifying in many regards. It is difficult to fathom.
