You may have previously travelled to the U.S. under the Visa Waiver Program (VWP) by completing the Electronic System for Travel Authorisation form (ESTA).
In January 2016, the U.S. implemented changes to this program. Travellers who formerly qualified for the VWP are no longer eligible to travel to the U.S. under it if they are nationals of Iran, Iraq, Sudan or Syria or if they have travelled to any of those countries on or after 1 March 2011. These travellers will now need to apply for a visa by following the usual immigration process at U.S. embassies and consulates.
From 21 January 2016 current ESTAs will be revoked for travellers who have dual nationality. This could have serious consequences if you have already booked U.S. travel as you will not be able to enter the country until you obtain a valid U.S. visa which could take a number of months. To check your status go to https://esta.cbp.dhs.gov/esta/.
It is possible that in certain circumstances the restrictions may be waived on a case-by-case basis. If you need a U.S. visa for urgent business, medical or humanitarian travel, U.S. embassies and consulates can process applications on an expedited basis.