Originally Posted by
Vulcan
I was under the impression that if you were a US citizen with a US passport, you were 'required' to enter/leave the US using your US passport. Was I wrong?
Yes, you enter with your US passport. But, when you leave the US by plane, there is no border control. UA and all other airlines do check that you have a passport as they can be heavily fined if you land in the other country with no passport (and In Germany they are heavily fined if your passport is not signed before presenting it to the border agents). But you can enter in the other country with another passport.