Ohrdruf was the very first concentration camp that was liberated by the US. It wasn’t until then that the US realized exactly how bad things were for the jews. When soldiers liberated the camp, they were met with piles of bodies. Those who were left alive were starved, scarred, and otherwise in bad shape.
In the book, The Diary of Anne Frank, you can see how bad jews were treated. They were marked with yellow Stars of David to let the rest of Europe know that they’re jews and should