Better Safe than Sorry - Apply early for ESTA

All nationals or citizens of Visa Waiver Program (VWP) countries who plan to travel to the United States for temporary business or pleasure and do not have a valid visa require an approved ESTA prior to the start of travel. Whereas the registration should be made at 72 hours before travel at the latest, applications may be submitted at any time before travel.

If ESTA is not approved, it doesn't mean you cannot travel to the US. It just means you’ll need to apply for a visa in person in Berlin, Frankfurt or Munich. Visa application processing takes seven to ten work days. 

Visa Waiver Travelers are not required to have specific plans to travel to the United States. ESTA registration is valid for two years.

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