Iran want to deal oil for Euro, right ?
https://www.reuters.com/article/us-oil-iran-exclusive-idUSKCN0VE21S
It means less demand for USD, and the USA just can't imagine a world where their currencie is not THE currencie. It's a bit stupid knowing that it means any country with some oil could indirectly control the value of USD. But the USA have the biggest army of the world, so ...