Originally Posted by
DirektornSE
The EU261 applies to operating air carriers only. For example, you purchased your tickets from Lufthansa, an EU airline, for a flight to Europe from the United States. However, the operating air carrier is its codeshare partner, United Airlines. Since the flight is on a non-EU carrier and is terminating in the EU, it would not be covered under EU261. If your flight was operated by Lufthansa, regardless of whether you purchased your ticket from United Airlines or Lufthansa, your flight would qualify under EU261.
https://consumerrescue.org/guides/eu...e-europe-trip/
But I'm happy to be corrected, I do not live in the US and have only once in my lifetime used EU261
OP speaks about the RETURN trip, it's still a US originating flight on a non-eu airline. I do not know if that qualifies for EU261
The fact that the entire trip started in the US does not matter. For the purposes of EU261, the outbound and inbound directions are separate events (each direction has its own origin and termination point). If you fly outbound US>EU EU261 applies if the operating carrier is EU based (not if DL). On the return EU>US (which is the OP's case) EU261 applies regardless if DL or KL is the operator.
(Also this point that EU-originating flights are covered even on non-EU airlines was not in the original EU261 rule. It was added/amended later based on court rulings)