Our leadership is unbothered because they, unlike the the rest of world's public opinion, and world leaders, except for a few sane ones, are well aware of our own abilities. We DO NOT want war but now that they've imposed war on us we will fight!
We're not afraid of the US and it's threats, there are capabilities reserved for them and their involvement will not change anything to their benefit.
What the US offered wasn't an equitable deal. It was surrender and surrender would've eventually meant a slow defeat and death. A compromise deal would have been better than war and would've spared people. Iran has ALWAYS avoided war because of this and not because of any fear.