Did I miss something? When were nuclear weapons used in Iraq?
Anyway, to answer OP's question about when nuclear weapons are justified, I think it's the same as the answer to asking that question about any other weapon: when the damaged by not using them is greater than the damage from using the weapon itself.
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"If you cling to something as the absolute truth and you are caught in it, when the truth comes in person to knock on your door you will refuse to let it in." ~ Siddhartha Gautama
"If you cling to something as the absolute truth and you are caught in it, when the truth comes in person to knock on your door you will refuse to let it in." ~ Siddhartha Gautama