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.
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).
