“Germany has updated its travel advisory for the United States to emphasize that a visa or waiver allowing entry does not guarantee Germans to enter the U.S. after several of its citizens were detained at the border.
According to an advisory on Germany’s Foreign Office website, the country warned that entry through the Electronic System for Travel Authorization (ESTA) system or a U.S. visa is not guaranteed every time and that the U.S. border control has final decision.
“It is recommended that you bring proof of your return journey upon entry,” the office said.”
https://www.yahoo.com/news/germany-updates-us-travel-advice-012325804.html