Originally Posted by woodenshoe101

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)