The whatever it is a sign/symbol/serves as a reminder (warning) , a memorial and remembrance

These three words can all be translated remember or commemorate, but there are different nuances. Mahnen has to do with admonishing, gedenken relates to thinking of - bringing to mind - erinnern can be similar to gedenken, but also means to remind someone.
How the monuments are called gives some of the nuances:
A Mahnmal is a memorial or monument commemorating always something negative, evoking sadness, regret, etc.
Gedenktafel is a memorial plaque that - could be negative - but also could honor or commemorate positive events/heroes.
Erinnerungsausstellung - likewise could go either way - a commemorative exhibition.

For once, this sounds better when presented as a noun phrase. See example below:

"Witnessing Genocide: Vigilance and Remembrance at Tuol Sleng and Choeung Ek"

The words are simply collected from the other suggestions, but I think this puts them into order. It's like a train of thoughts:

admonish - is the warning which starts with "don't you remember when..."
remember - is not as strong as "commemorate" (IMHO) but the first step towards it
commemorate - wants to preserve the memory of something, therefore finishing the admonishment

btw Great suggestions everyone!

A late suggestion, but maybe it helps! The imperative form might actually work well here.

Similar context:

“Directly connected to these ideals are the way visitors are able to reflect, remember and recall the historical significance of the Holocaust through the United States Holocaust Memorial Museum.”

http://www.history.ucsb.edu/projects/holocaust/Research/Pros...

It is rather difficult to translate this somewhat understated commentary on Nazi terror into an English context.