H.R.158 - Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015 introduced in late 2015 suggests that travellers eligible for the visa free travel to the US under the visa waiver program (by simply arranging an ESTA online), will no longer be able to travel visa free if they have visited Iraq, Syria, Iran, Sudan, Libya, Somalia, or Yemen in the last 5 years. Full details of changes can be found here https://www.congress.gov/bill/114th-congress/house-bill/158/text.
As of the 1st of January 2016 new questions were added to the ESTA application in regards to previous travel history. Stating that you have previously (after March 2011) visited Iraq, Syria, Iran, Sudan, Libya, Somalia, or Yemen will make you ineligible to travel under the Visa Waiver program.
This does not, however, stop one from travelling to the USA altogether. Travellers no longer eligible for a visa free travel will need to apply for a visa in person at the embassy of the US in their country of residence.
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