We are still in a cold war, only now it's more complicated because more countries have nukes...including North Korea which is kind of a wildcard.

Check out Nuclear War by Annie Jacobsen. It's a well-researched and horrifying scenario where deterrence fails because N Korea launches on the US. it takes you step-by-step through the decision process our government would take in response. Its gripping and pretty damn terrifying and shows just how delicate the whole situation is.