One of Obama's biggest accomplishments was negotiating a deal to stop Iran from building nuclear weapons. The reasons countries want nukes is not unknowable or unhinged; it's coldly pragmatic. While conventional armies are hugely expensive, nukes are cheap. No country could hope to build a conventional war machine that can compete with the US's. If they fear this large, menacing hegemon that routinely bombs or invades foreign countries, the quickest and most effective check is nukes. All of this perfectly basic, obvious calculation.
The cornerstone in the Iran deal was trust. Over years, Obama was able to convince Iran that the US would not behave capriciously. The US convincingly communicated that the reason Iran might want nukes--the threat from the US--was smaller than the pain of sanctions. Again, pragmatically transactional thinking.
Enter Trump, a lunatic with a middle-schooler's understanding of geopolitics.. Just by threatening to blow up the deal, he demonstrates that the US is no longer worthy of that trust. The US now looks like a dangerous, unpredictable actor. Unless things change quickly, the GOP's jejune lust to appear "strong" will probably convince Iran to restart the program. Any rational country would. And the truth is, they probably should. Even reasonable diplomats couldn't plausibly argue that the US is worthy of trust. We're just not. And other countries will begin to take action that reflects their well-placed mistrust of the US.