In which military alliance was the GDR a member?

EDITORIAL LEARNING AID

Why is this answer correct?

From 1955 to 1990 the GDR was a member of the Warsaw Pact. NATO was the Western alliance, the Confederation of the Rhine existed only from 1806 to 1813, and there is no military alliance called the "European Alliance".

GDR membership: Warsaw Pact, not NATO.

This translation and explanation are editorial learning aids. The official German wording and answer key govern the examination.