[travisby]

I think a good example for your hypothetical would be the sinking of a nuclear plant propelled ship. If country B invades the territorial waters of country A with a nuclear destroyer and country A destroys it, the fallout (morally and chemically) would still be on the aggressor for creating the situation where it's reasonable to expect their harmful payload would be destroyed.

[prawn]

I'm imagining something far less obviously threatening (balloon rather than a destroyer) and far less explicitly damaging (maybe a liquid or gas that would mix with the missile impact, and rain down barely noticeable granular substance that, oops, contaminates farm land).

I can appreciate this is quite far fetched given that the material would need to drop such a distance rather than completely disperse with wind, and not be super obvious about it (e.g., pellets).