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1. The U.S. no longer maintains a command post on continuous airborne alert. There are two planes on 15 minute ground alert, but they are very vulnerable sitting on a limited set of alert facilities.

2. A couple of new submarine detection methods have gotten some development. Signal processing for sonar has improved.

3. Missile accuracy has improved, particularly in a first strike scenario where satellite navigation would be available.

4. Aerial drones have gotten much more sophisticated and compact, which could jeopardize silos, communications infrastructure, bombers on the ground, and the E-6s and E-4s.

5. Sea based drones have undergone extensive development and might become a serious threat to the submarine force.

6. Anti satellite weapons have advanced significantly, putting communications satellites to transmit launch orders at risk.

7. There has been more work on maneuvering reentry vehicles and hypersonics, which could enable depressed trajectory launches which, if they were accurate enough to destroy silos or the underground launch control facilities, could knock out the ICBMS with less than 10 minutes of warning, which is not enough to retaliate in time.

The historic red lines in the cold war were either one side reaching a first-strike position, or an unstoppable ground invasion in which both sides soldiers were directly involved and tactical nuclear weapons would usefully repel the invasion. Even then NATO troop deployments in Germany during the cold war reflect both a risk that this tipping point wouldn't trigger a nuclear war - as well as carefully calculated strategies to ensure that the risk of a nuclear escalation was high such as by stationing tactical nuclear weapons in Germany.

It's difficult to identify a similar red line in a likely conflict between any of the major nuclear powers.

World powers can’t directly fight anymore so Russia takes chunks of Europe every so often. China does the same. Ukraine and Taiwan, for example, lose out. We pretend there is peace.

But it was also extremely abstract. My teachers talked about their memories in the 50s and 60s. Works about the bombings in Japan were in literature. Nothing tangible. It was pretty fatalistic too. It probably wasn't going to happen, and seems less likely to happpen. But if it did happen there wouldn't be much after. I do remember my science teacher wondering, as if aloud to himself, if the windows in the classroom would offer meaningful UV shielding before they melted.

It felt like it was in the immediate past, most of the time. In hindsight more of a cliffhanger than a resolution.