Do I have to spell it out? OK. A 'credible threat' is only credible if you are prepared to carry it out. In other words, if the US wants to present a 'credible threat' to Iran, it has to be ready to fight, if Iran ignores the credible threat. Sane countries don't like to make threats they aren't prepared to back up, because failing to carry out a threat makes you look weak. So, unless the US wants a war with Iran, it should not be making 'credible threats' to it. OK?
from the article: U.S.: Force is not the only way to stop a nuclear Iran