Quicksand is a type of loose sand or soil that cannot support weight. The expression "upon quicksand" plays on the characteristic of quicksand to "swallow" things that fall on it. Because of this quality, quicksand is a terrible foundation upon which to build something. Therefore, the difference between a permanent peace and one built "upon quicksand" is that the former is likely to be long-lasting, while the latter has weak foundations and is unlikely to last very long.
