Who were the situationists?

The first issue of the journal Internationale Situationniste defined situationist as "having to do with the theory or practical activity of constructing situations. One who engages in the construction of situations. A member of the Situationist International". The same journal defined situationism as "a meaningless term improperly derived from the above. There is no such thing as situationism, which would mean a doctrine of interpretation of existing facts. The notion of situationism is obviously devised by antisituationists."