There are indeed more players in the game, and what makes for deterrence is a good question. Nuclear annihilation is pretty deterring, and it probably will keep Iran, for example, pretty much in line for some years to come, nukes or not. The United States also has overwhelming conventional force, which is also a deterrent.

The deterrent we understood during the Cold War was basically a two-player game, which is much easier to analyze than multiplayer games. But a lot of today’s faceoffs are two-player: Iran-Israel, India-Pakistan, India-China, Russia-neighbor (none of whom have nukes, but some of whom are in NATO), North Korea-whomever. Some of these, particularly the India-Pakistan-China tangle, might be considered more than two-player.

It seems to me that that last is the most dangerous situation. Pakistan is unstable, and Afghanistan complicates things. China, fortunately, seems more interested in making money than in making war, but might feel compelled to enter a serious disagreement between Pakistan and India.

Israel-Iran-US is another multiplayer situation, but my current guess is that nobody’s going to attack Iran. And Iran doesn’t have nukes and, if it wants them, won’t have them for some time, so it’s not going to be using them on anyone.

I think that today’s complicated situation hasn’t really been analyzed in terms of deterrence. Most of what I see that uses that word is simply dragging it in with its Cold War meaning, now pretty much obsolete.