Using even a small nuclear weapon against Iran would be bad news for global stability, risking a cascade of nuclear use elsewhere in the world.

• A US or Israeli nuclear strike on Iran, even a small "tactical" one, is seen as very likely to push Russia toward using a nuclear weapon in Ukraine.
• The fear is a domino effect: nuclear weapons could then come into play between North Korea and South Korea, or India and Pakistan.
• No nuclear weapon has been used in war since World War II, and breaking that taboo is described as opening a Pandora's box that can't be closed again.
• Advisers telling the president that a modern nuclear strike would be more "surgical" and low-casualty than the WWII bombings are warned to be dangerously wrong.

Outlook: Pressure is growing on Iran to negotiate to avoid a strike, but if one happens anyway, expect sharply higher risk of nuclear escalation in other conflicts worldwide.