What promise did Trump break with Iran?
It's not about a promise that Trump broke, it's about a promise that the US broke. The USA is still the USA regardless of who's in charge.
Why would any other country in their right mind want to make a deal with the US when, come next election, everything they were promised could go up in smoke? No matter what you think of the Iran deal, moves like this hurt our credibility.
That was a deal that Obama made...not the US Congress. It's why it's an agreement and not a treaty. As such Trump has zero obligation to honor a deal that was awful for our interests and should have never been signed.
Doesn't matter that it was Obama. Obama was representing the US when the deal was made. Therefore, backing out of the deal makes the US look bad.
We can try to renegotiate, but straight backing out of the deal ruins our credibility.
Obama was representing himself when the deal was made. Backing out of the deal makes Barry look bad because the deal was bad...not the US. To be blunt...the US looked bad when Barack Obama signed such an awful agreement in the first place! What Donald Trump has done is to restore some semblance of credibility to the US.