Fred Saberhagen's "Mask of the Sun" is at times classic sci fi time travel, strong alternate history and richly woven historical fiction. The premise is based on the existence of certain goggles that allow the wearer to see events from the future. But it only works sometimes, and it's unclear what it chooses to show the wearer and why. Saberhagen uses this premise to open a multi-centries-fun ride built around the early conquests of the New World.
After his brother discovers a mysterious jade mask near the Florida keys and then disappears, Mike Gabrieli is pulled into a sordid conspiracy and out of time into a far future world that pits 23rd century Incas against 23rd century Aztecs. The future-seeing goggles were turned into a religious artifact by the Aztecs before Montezuma sent it to Hernan Cortes as a gift, and then re-gifted it to Francisco Pizarro as a farewell charm on his way to conquer Peru. Gabrieli becomes part of a 20th century commando team training to defeat a 16th century Pizarro at his foundational battle with the Incas at Cajamarca shortly after he arrives in Peru.
The 23 century Incas and Aztecs are not the same as their 16th century counterparts. In a historic timeline in which Pizarro did not conquer Peru, the Incas grew to world prominence expanding their already vast South American empire, and not without a little help from superior technologies. The Incas and Aztecs are world empire adversaries. Each are able to pop through time to advance their respective causes. While each is certainly more advanced, the Aztecs still sacrifice individuals and share the sacrificial parts as food (although it's handled in a much more sanitary and efficient manner than in the past). It's a little unclear how "good" the new age Incas are, but they seem to be more good than the Aztecs.
Saberhagen's futuristic landscape is rich and imaginative and his historical descriptions are filled with well-documented details and fleshed out with a master's flair for color. Mike's commando team must learn the ways of 16th century Peru and incorporate their knowledge of history in their plans to change it.
For much of the time, the Mask gives Gabrieli only solitary and unmoving views that Saberhagen uses as foreshadowed clues to a future event. At other times, the Mask gives Mike a moments-ahead view that he uses to escape from one scenario and get to the next. Written poorly, it could leave readers very confused and turned around, but Saberhagen deftly provides an interesting visualization.
In this short novel (just over 200 pages), Saberhagen spends very little ink on the particulars of time travel - suffice it to say that he goes with a multiple timeline theory that provides for the existence of a world where Pizarro defeats the Incas and a world where he doesn't. I was reminded of Stephen Baxter's multiple timeline theories he writes in his "Time Machine" sequel - "Time Ships".
I also particularly enjoyed Saberhagen's spare and dramatic use of the themes of coincidence v. predetermination - where the slightest change in behavior or physical action could cause some near term or long term impact. Orson Scott Card addresses a similar theme in his "Pastwatch: Columbus" novel, where a future world society isolates many of the worlds problems to the global slavery crises created by Columbus' discovery of the new world. Card's handling of the theme is, however, more poetic and deep.
"Mask" contains spaceships and ray guns and there's no doubt about it's sci fi core. However, there are lengthy battle scenes which pit Pizarro and his Conquistadors against the Incas, and post-conquest interactions between real and imagined Spaniards and Incas which provide more than a hint of historical fiction/alternate history. The thing that excites me the most is that Saberhagen places his action in the midst of two of the greatest cultures the World has ever known, and two which the worlds of sci fi and alt/historical fiction rarely touch. It's a bold and innovative world.