War time is an especially bad time for art, libraries and archives, either because of the ensuing chaos, destruction and opportunism, or the systematic plundering and destruction of the invaded territory. As an example, take the truly staggering amount of art and cultural artifacts looted or destroyed, often systematically, during WWII by the Nazis and the Soviets. The Polish Ministry of Culture and National Heritage, for example, has a division[0] that specializes in cataloging and attempting to recover such looted art. It is interesting how many suriving works are located in known private or public collections abroad, even periodically showing up at auction. ArtSherlock[1] (in Polish) also has some interesting articles about some of these works. They used to maintain a smartphone app that let you take pictures of artwork to determine whether it is a known missing piece, and then report it, if it is.

[0] http://dzielautracone.gov.pl/

[1] https://artsherlock.pl/