Three Genres In One Series

In Time Patrol:

we see Manse Everard with Carl and Laurie Farness in Hawaii in 43 A.D. (science fiction);

we see Ermanaric kill himself in Gothland in 374 (historical fiction);

we read an imaginatively reconstructed earlier stage of Northern European mythology (fantasy/mythological writing);

we see barbarians waging war against the Roman Empire (historical fiction);

we are pleased to meet Everard again in Amsterdam in the late twentieth century (science fiction).

For over a decade, the Time Patrol was four short stories collected in a single volume. Then Poul Anderson expanded it beyond anyone's expectations.