I sometimes used to get an alarming amount of grief entering the US because I had quite a few Turkish stamps in my passport as we used to go there a lot on holiday. I hate to think what would happen with an Iranian stamp!

The US government is very concerned about people who use Turkey as an entry point to nearby conflict zones in Syria and Iraq. When I applied for the Global Entry program the only interview question they asked me was why I had visited Turkey.

I have an Iranian visa in my passport, so I had to get a US visa to go to the US. I didn't have to talk to anyone when entering — the electronic thingy scanned the visa, and that was it.

I have a full page visa and entrance stamps from Sudan in my passport, and I've crossed into the US three times now with that in my passport and they've never said a word.

Seems you're not eligible to a Visa Waiver if you traveled to Iran or Sudan. Which means you need to apply for a visa at your US embassy [1] (which in my case was expensive and required a one day trip). Also it's not only a matter of having an Iranian stamp on your passport. You're not eligible for the VWP even if you visited this countries. I wouldn't try to lie about this!

"When can’t I use the VWP (ESTA) and need a visa?

Due to personal circumstances:

you traveled to certain countries (Iran, Iraq, Sudan, Syria, Libya, Somalia, and Yemen on or after March 1, 2011)"

[1] https://nl.usembassy.gov/visas/visa-waiver-program/

I think the issue is that Sudan has a very different dynamic with the US with respect to military intelligence/affairs. I don't really think Sudan is considered a sophisticated threat at the moment.

Not to mention, they appear to be going through a transitional stage towards democracy.

I think they (Iranian government) recently changed the rules and don’t stamp the visa into the passport. Not sure though, you better do a little research about it.

Went to Iran a few years ago and my esta got declined.

And I'm from Germany...

I don't think I would not get a visa at an embassy.

Just get a fresh passport after the trip, with no Iranian stamps. You should be fine unless you are person of interest to intelligence agencies.

This is a bad idea. Just get a visa. I did that and had no issue with getting the visa nor entering. Do not try to break immigration law — you can easily get permanently banned from entry if you lie to immigration officials.

Of course, don't lie if you are asked but there's nothing illegal about getting a new passport. Better not have a stamp of a country that is in very bad relations status with the country you are going to visit. Iran is not the only example here.

You still have to answer the question "Have you been to Iran?" when applying for the visa waiver program.

Getting a second passport is basically standard procedure, why would that get you banned from entry?

Getting the new passport is not the issue. You have to answer the question "Have you been to Iran?" when applying for the visa waiver program, and this application is what you shouldn't lie on.