Of the 107,000 Jews living in the Netherlands before WWII, all but 5000 were deported by the Nazis to concentration and death camps in central and eastern Europe. Their fateful journey began at Kamp Westerbork, a Nazi transit camp in a forest 10km south of Assen, near Hooghalen village. An excellent on-site museum traces the history of the holocaust in the Netherlands, and photo blow-ups mark the locations of Nazi-era features such as the punishment building and workshops where internees worked as virtual slaves.
Kamp Westerbork, ironically, was built by the Dutch government in 1939 to house German Jews fleeing the Nazis. From 1942 the Germans used it as a transit point for those being sent to death camps, including Anne Frank.
The camp itself is a 2km walk through a forest from the museum (or a €2 bus ride). There is little to see here today: after the war the Dutch government used the barracks for South Moluccan refugees, and then had them demolished. Listening to voice recordings of inmates recounting life at the camp (taken from recovered diaries), it is not hard to imagine the distress and horror of the detainees.
Of several monuments standing here, perhaps the most moving is at the roll-call site in the centre of the camp, with 102,000 stones set upon a map of the Netherlands.
The camp is 7km north of Westerbork town (and 7km south of Assen). There is no direct public transport to the site.
By train, use the station at Beilen on the Zwolle–Assen train line (at least two trains per hour), then take bus 22 to Hooghalen, from where it's a 25-minute walk east. By bike, head south from Assen via the rural knooppunten 9-8-33-31-58 cycle route.