[a_humean]

Due to how interconnected European and American finance and business is this effectively means that Europe cannot trade to Iran.

The Europeans cannot enforce the JCPOA without either some kind of American cooperation or a major decoupling their financial systems from the US. I believe Macron has made public musing about the latter as an option, but then again Macron says a lot of things that lack pan-european support.

[tomatocracy]

Specifically, the US imposes/threatens to impose secondary sanctions ie anyone who does business with Iran is also supposed to be subject to sanctions by the US, even if they are not US companies/US parent companies. So those companies can't then themselves do business with US companies or in the US.