Ara Pacis Museum, Rome, Italy

Here are a couple of websites with some history of the Ara Pacis:

• Wikipedia Ara Pacis page - as too often the case with the Wikepedia, the article is a little thin and a little sketchy on the facts.
• Ara Pacis page maintained by someone at the University of Chicago from "A Topographical Dictionary of Ancient Rome", Oxford University Press, 1929.