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I have had a green card since 2013. I left the United States on the 2nd of January. I have both German and Iranian citizenship. I was in Iran in August 2016.

Can I return to the United States without a problem?

And when, at the latest, can I go back to the USA?

Away from the U.S. less than 180 days, no problem. Between 180 days and a year, maybe a problem. More than a year, definitely a problem.

You should plan to return home (the U.S. is your home, right?) before the 1 year mark, and be prepared to explain why you were away so long and show evidence that you have not abandoned your residency.

It's important to note that CBP might try to pressure you into signing an I-407, which essentially is you admitting that you've abandoned your residency. You do not have to sign this - as an LPR you have the right to be admitted or paroled into the country pending a court hearing where you and your lawyer can show that you did not intend to abandon your U.S. residence.

Then again, it's reasonably likely that you'll be admitted without any hassle (it all depends on the CBP inspector and what she had for breakfast that morning) so try not to worry too much.

References: The legislation, regulations, policies and case law in this area are rather complicated and easy to misinterpret, and IANAAILAL. However, the 180 day threshold comes from INA § 101(a)(13)(C)(ii), after which an LPR alien is "regarded as seeking admission". The 1 year threshold comes from 8 CFR § 211.1(a)(2), after which an SB-1 returning resident visa is needed, which is not easy to get. Further discussions: 1 2 3

The USCIS Maintaining Permanent Residency does not give any timelines on when they would consider someone to have abandoned their permanent residency status in the US.

Some items that could cause that include taking up residency in another country, not filing tax returns, not maintaining a residence (house/apartment) in the USA, etc.

Another USCIS page, International travel as a permanent resident states that brief travel is fine, but as a general guideline, being outside the USA for over a year will cause them to a look at the circumstances of the travel and whether you have abandoned your permanent resident status. As already stated, items that can you can use to prove that you haven't abandoned your status are keeping US employment, address, ties to family and friends in the US, and so on.

One way to show that you do not plan to abandon permanent residency is to apply for a re-entry permit/travel document before you leave the US, Form I-131.

An extended absence from the USA may also mean that you cannot count that time when applying for citizenship, so it may delay your citizenship application.

AIUI the new rules on Iranian citizenship and visits only pertain to VWP travel. They should not be an issue for permanent residents.

The bigger problem is that by spending too much time outside the US or by taking up residence abroad you may be deemed to have abandoned your permanent residence. According to http://www.familytousa.com/reentry-permit/ there is an automatic cutoff after one year unless extra paperwork is filed but in some circumstances even a year may be too much.