Are there entry requirements for layover airports between the USA and SE Asia?

US citizens traveling from the USA to Ho Chi Minh City via Taipei, returning from Bangkok to the USA via Frankfurt. Will not be leaving the airport in Taipei or Frankfurt.
What documents are required to pass through these airports?
(Documentation for the destination countries is understood, only asking about the layover airport countries.)
Best Answer
US citizens may visit Taiwan for up to 90 days without a visa. Passports need to be valid during the entire visit.
US citizens may visit the Schengen States, of which Germany is part, for up to 90 days per 180-day period without a visa. Passports must remain valid for at least three months after departure; this rule also applies to airport transit.
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