Trump and Iran sanctions-what real effects?

In My Humble Opinion
1

One thing I don’t understand about the Trump and the Iran deal-I don’t expect any other country to follow the US in abandoning the deal and so how big a deal will Trump be? I realize that the US is central to international trade and when the US Treasury imposes a ban, all banks that wish to do business in dollars or with a US bank (essentially all banks) have to follow along. But what would happen if the Bank of England, and central banks all over the world, didn’t follow along? Could they stand up to the US Treasury and get away with it?
The US doesn’t do any significant trade with Iran itself now, so a new embargo isn’t going to effect that.
The agreement has a clause to automatically impose sanctions if Iran is caught cheating-but says nothing about what happens when the US bails. So I am assuming none of those sanctions will come into effect.
The UN repealed their sanctions, so no other country is required to do anything.
I am sure the US can cause significant problems for Iran, for instance prohibiting any global US oil firm from trading in Iranian oil-but there are other oil firms out there that could pick up the business.

Assuming none of the allies follow the US lead, how much pain will the US imposing sanctions on Iran cause Iran? Might it be a net gain for Iran? They will be able to open up new trading deals that are unblockable by the US.

Anyone following this issue and care to explain?

2

I found a significant part of my answer:

It isn’t just oil companies, virtually all multi-nationals do more business in the US (or simply sell items with US parts) than they can afford to do without. Boeing and Airbus are two big examples.

It still doesn’t answer the question of what would happen if the various national Gov’ts told the US not to interfere with their national interests. European countries are reportedly seeking exemptions to prevent their companies from having to cancel existing contracts. But what happens to new deals? Will the rest of the western world toe the line or would they seek formal ways around it?

3

The speculation that Trump may re-impose sanctions against Iran, is one the key reasons that the price of oil has shot back up in the past few weeks above $70 per barrel.