Germany does not favor dual citizenship. German citizenship expires by operation of law at the moment the citizenship of another country is accepted. This rule deprived many Germans living in the U.S. of German citizenship.

If you qualify for an exception you are eligible for a so called Beibehaltungsgenehmigung (permit to retain German citizenship), which must be obtained before accepting U.S. citizenship. The Beibehaltungsgenehmigung has opened the path to dual citizenship for many Germans living in the United States.