The two countries would be fighting for the simple fact they exist. Or, for the simple fact Trump is president of USA. That's how Trump is, and that's how the two countries are with each other.
But, Trump pulled out of the nuclear deal. Iran was angry. USA sanctions Iran. Iran is angry. Iran makes threats. USA sends troops. Iran gets even angrier. So, Iran threatens war. USA threatens war in return. (And, the entire planet knows USA will win, just from sneezing, even if the entire planet goes to war against USA with Iran). So, these tensions build up. USA and Iran have been enemies since 1979. That's not going to chance over night, or at the snap of a finger, or just because someone said for it to be so.
That's what's happening.