A Multiversal Threat?

Poul Anderson's Operation Chaos, four stories unambiguously set neither on our Earth nor on another planet but on another Earth in an alternative timeline, does not mention multiple timelines (I don't think) except in the Introduction written for the collected edition. Three Hearts And Three Lions begins on our Earth, then moves to another. Finally, A Midsummer Tempest provides an inter-universal meeting place for travelers from many timelines. Thus, there is narrative progression.

In that Introduction to Operation Chaos:

"Whatever manifold form it takes, the war of Law and Chaos surely goes on in them all.
"We have learned certain things. We ought to broadcast the lesson and the warning."
-Poul Anderson, Operation Chaos (Sutton, Surrey, 1995), p. 2.

Law and Chaos make war in Three Hearts... and in Michael Moorcock's Multiverse. Order and Chaos make war in DC Comics, including Neil Gaiman's The Sandman. So what is the warning? Will Chaos attack all the timelines simultaneously? Will characters from different Anderson works and from other fictional universes have to join forces against a common threat? Well, no. Anderson was an imaginative but also a restrained author who continued to write good novels of different genres set in different periods and universes but who did not follow up on that Introductory warning. Its sense of impending menace contributed to the particular volume but did not need to be taken any further.