Recorded1961-08-13dateinfrastructure

The Berlin sector border is sealed

Overnight on 12–13 August 1961, East German forces closed the sector boundary in Berlin with barbed wire and began building a permanent barrier, ending the last open crossing point between the two systems in Europe. The immediate driver was emigration: the loss of population through Berlin had reached a rate the East German state could not sustain. The western powers protested and did not intervene, having concluded that their essential interests were access to and the freedom of West Berlin rather than free movement across the city, and those were not being challenged.

Historical — it happened, and the record is settled.

Subjects

The Berlin Wall · linear fortification system · Germany · not located
Berlin · city · Germany

Evidence

Partly verified — Core facts are sourced; some optional detail is deliberately absent.

Chronik der Mauer and the victims-of-the-Wall research
Named as the standard research documentation for the Berlin Wall, the events of 9 November 1989 and the contested count of deaths at the Wall. V55 verification basis: NOT consulted. This pack therefore states the death toll as a disputed range attributed to differing counting criteria rather than as a figure, and treats the timing of the Bornholmer Strasse opening as unchecked.
Stiftung Berliner Mauer, Zentrum für Zeithistorische Forschung Potsdam and Bundeszentrale für politische Bildung · Partly verified
Chronik der Mauer, the joint documentation project on the Berlin Wall, and the associated research on deaths at the Berlin Wall 1961–1989

Recorded on The Berlin Wall

1948-06-24 – 1949-05-12The Berlin blockade and airlift
1961-08-13The Berlin sector border is sealed
1961-10-27 – 1961-10-28American and Soviet tanks face each other on Friedrichstrasse
1974The first Berlin Marathon, in the Grunewald
1981-09-27Berlin moves its marathon into the city
1989-11-09The Berlin Wall opens
1990-09-30The Berlin Marathon runs through the Brandenburg Gate

Related because they share a subject in the Atlas — never because the text looks similar.

The Berlin Marathon runs through the Brandenburg Gate · 1990-09-30
The Berlin Wall opens · 1989-11-09
Berlin moves its marathon into the city · 1981-09-27
The first Berlin Marathon, in the Grunewald · 1974
American and Soviet tanks face each other on Friedrichstrasse · 1961-10-27 – 1961-10-28
The Berlin blockade and airlift · 1948-06-24 – 1949-05-12