Why, in the below example is the imparfait used?
"Il y a cinquante ans, le 27 janvier 1945, l'armée rouge libérait le camp" - Le Monde, 1995
Shouldn't it say "l'armée rouge a liberé le camp", since it is a completed action? Or yet use the passé simple: "l'armée rouge libéra le camp"?
Why is the imparfait used in this example?