The United Colonies Triumphant was an American scientific romance depicting an alternate history in which the British colonies in North America had gained independence in the 18th Century. In the 20th century, the former colonies came to the rescue of the British Empire, which was engaged in a major European war against, improbably, a united Germany under a single malign ruler.
Colonel Thomas Bushell found a copy in the Sons of Liberty hideout in Buckley Bay and took it along on his trip to The Six Nations in an attempt to gain insight into their way of thinking. He found it unscientific, not at all romantic, and written by a poor author who went to great lengths to prove his inability to write. Bushell also considered the idea of Germany being united under a single malign ruler to be absurd. Other people considered the novel to be entertaining, but rather far-fetched and improbable speculation.
The reference to an inept alternate history writer may be a self-deprecation by author Harry Turtledove.
While the novel's plot strongly resembles the history of OTL, it is unclear whether it deals with World War I or World War II, as the "single malign ruler" of Germany could refer (from the British perspective) to Kaiser Wilhelm II just as easily as Adolf Hitler. Further, in the novel slavery continued to the present day so it is not completely the same as OTL.
"Scientific romance" was the term H.G. Wells used around 1899 to describe the genre of his speculative novels including The Time Machine and The War of the Worlds. It swiftly lost out in everyday vernacular to "science fiction".