Moving across the nuclear threshold wouldn’t necessarily mean an immediate, full-force nuclear exchange – in other words, global thermonuclear war. But it would be an extremely dangerous, watershed event in world history.

The nuclear option that has been most frequently discussed in the past few days involves Russia using a small nuclear weapon (a “non-strategic nuclear weapon”) against a specific military target in Ukraine. Such a strike might have a military purpose, such as destroying an airfield or other military target, but it would mainly be aimed at demonstrating the will to use nuclear weapons, or “escalating to de-escalate”, and scaring the west into backing down.

Some analysts have questioned Russia’s ability to actually carry out such an operation, given its lack of practice. Unfortunately, this isn’t the only or even the most likely option available to the Kremlin. Based on war games I ran in the wake of Putin’s 2014 invasion, a more likely option would be a sudden nuclear test or a high-altitude nuclear detonation that damages the electrical grid over a major Ukrainian or even Nato city. Think of an explosion that makes the lights go out over Oslo.