score:40
The Majdanek concentration camp was liberated by the Red Army on July 22, 1944. Since it was a largely intact structure, it took the joint Soviet and Polish commission only two month to prepare a comprehensive report, and by autumn, the site was declared a historic monument. In November 1944, a museum opened on the grounds, the first one "devoted to the commemoration of the victims of World War II in Europe". It seems the first permanent exhibition was opened in 1945.
As early as August 1944, journalists were invited to visit the site. For example, on August 29th, Life magazine published a short illustrated article describing a funeral ceremony three weeks earlier.
In 1947, the Polish parliament established both the Majdanek site and the Auschwitz concentration camp as "monuments of martyrology". Since 1965, the museum is a State museum.
There is a monography (in Polish, which means I can't read it) about the early history of the museum:
Upvote:6
As another contender, the border was closed between east and west Berlin on 12-13 August 1961 with the wall going up shortly after. According to Wikipiedia the Checkpoint Charlie Museum had its first exhibition on 19 October 1962, a little over a year later. This was in an apartment however, and the museum opened at its permanent location in July 1963.
I found this because I was wondering whether there were any museums that opened shortly after the fall of the wall - that still seems like a good place to look.
Upvote:37
The UK's Imperial War Museum was founded in spring 1917, nearly three years after the outbreak of WWI, when it was already clear that the war would have great historical significance. This was over a year before the war ended, and that ending was somewhat unexpected: in spring 1918, the Allies hoped to force a victory in 1919.
In spring 1917, the idea was proposed to the Prime Minister, Lloyd George, accepted, and a committee formed to organise it and collect material for exhibition. The committee visited the Western Front and consulted with the Commander-in-Chief, Haig. The first exhibition by the museum was at the Crystal Palace, in South London, and was opened by George V on 9th June 1920.
The original remit of the museum was the First World War. It started to collect WWII material from the outbreak of that war (and had to return some of its exhibits for use). From 1953, with the Korean War and the Malayan Emergency, its remit was expended to all modern conflicts in which British or Commonwealth forces were involved.