A lot of things US and other countries can do after the fact, as they did in the World War II, which stopped Hitler's advance, but did not do any good to millions of Jews who went to Hitler's gas-chamber. Strong reaction is not an option in case of nuclear bomb.
As you said, young Iranians are in favor of US, why would US decimate millions of its supporters in Iran. Besides, Mullahs are in power, not people of Iran. So, people should not be held responsible for what Mullahs do.
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