Such travelers must apply for a visa. If they attempt to travel without a visa, they may be refused entry into the United States. Under the Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015, those who have traveled to or been present in Iran, Iraq, Sudan, and/or Syria on or after March 1, 2011 (with limited exceptions for travel for diplomatic or military purposes in the service of a VWP country) OR travelers who are also nationals of Iran, Iraq, Sudan, and/or Syria are no longer be eligible to travel on the VWP.