The Deutsches Historisches Museum is Berlin's big German history museum, set in the old Zeughaus on Unter den Linden. Its collections range from early history to the present, with the strongest pull for many visitors being the way it connects empire, war, division, reunification, and modern Germany in one long story.
This is not a quick caption museum. Give it time, especially if you want context for the rest of Berlin rather than another isolated monument stop.