It is quite interesting, perhaps more to those not au fait with financial markets, how America can use its power to end trading relationships with a Country, globally.
The US will re-instate its sanctions on Iran shortly, on the same terms as they were before the Obama deal.
What this means is that all companies that have any US customer so subsidiaries will have to stop trading with Iran. Take Airbus for example, an order for 100 new planes from Iran Air.
But, Airbus trades with US too of course, so they can be sued in the US by the Government for sanctions breaking.
More critically this applies to Banks, if you have a US subsidiary then you can get fined for any kind of lending or transaction servicing for any deal to do with Iran or where there is an Iranian Party.
In 2014, BNP Paribas got a $9 billion fine from the US for 'sanctions busting' Of course in the US no BNP entity had broken sanctions it was a third party transaction between French and Iranian companies.
So US Sanction end a countries ability, thank to these extraterritoriality rules, to trade in effect with any Western Bank, but also most banks full stop. Iran will have to look for finance to Russian banks or other odd third-world banks. Without finance oil trading is very hard. One thing I have seen this week is the gold price increasing, gold can be used here as a form of finance, I have heard alleged that this is how Turkey manages to buy the ISIS oil and now Iranian or illegal Kurdish oil and avoid sanctions.
Anyway, all the posturing by Europe will come to nothing, America is the ruler of global finance and if it says stop then tough on you!