From my experience working with Americans, you could compare it to the damage to records which happened in the civil war and war of independence, or probably it was less. There are many local archives around Europe, and much of the damage was on a local level, meaning specific collections or bits of collections were damaged or lost. But for English speakers one of the biggest conflict-related losses of archives might be the Irish one during their revolution. I can't by the way think of any archive I've come across which was stolen deliberately during a war in any major way, but there were I think cases which ended up in private hands (often American), and effectively got lost to research that way.