Conquistador is a 2003 alternate history novel by S. M. Stirling. Its point of divergence occurs when the empire of Alexander the Great endures long after Alexander's death, creating a markedly different history that prevents the European conquest of the Americas. Most of the story is set in the parallel universe affected by this history.
Plot
John Rolfe VI is an infantry captain who comes back from World War II with a war wound and few prospects, but in 1946 a radio he is rewiring malfunctions and creates a gateway to a parallel universe. This universe is one in which Alexander the Great lived a full lifespan, creating an empire that stretched from Iberia to the Indian subcontinent. In this world, the Macedonian Empire proved so strong and durable that it redirected the barbarian migrations of the Goths, Vandals, and others eastward towards China and the rest of the Far East. As a result, what remains of China is a hodgepodge of Indo-European dominated states, the Americas remain undiscovered by the Old World, and technology has barely progressed to a medieval level. Deciding to take advantage of the untapped resources that await in this different California, Rolfe gathers members of his infantry company to help him explore and develop this new world. Over the next 60 years, he builds a new nation, which he calls the Commonwealth of New Virginia.
In 2009, two California fish and game officers, Tom Christiansen and Roy Tully, are trying to solve the mystery of how large numbers of pelts from endangered species are showing up. They finally deduce the secret of the gate to the parallel world, but before they can make the secret known to their superiors, they are kidnapped and permanently transported to New Virginia by Rolfe's granddaughter, Gate Security Agent Adrienne Rolfe (with whom Christiansen had been falling in love).
Once the two rangers get over their resentment of being forcibly and permanently removed from their lives and world, and being brought to this new world, Adrienne enlists them to help sabotage a coming coup in New Virginia. Giovanni Colletta, head of the second most powerful family, and son of a sleazy and amoral war buddy of Rolfe's, has resented the elder Rolfe's control, and he and some allies are planning to take over by force and violence, with the intention of imposing an authoritarian regime. The rangers decide that Rolfe and his allies are the lesser of two evils, and decide to help Adrienne in her effort to prevent the coup. The group discovers that Colletta is arming post-Aztec and post-Mayan Indians to build a couple of battalions of soldiers (something very illegal under Commonwealth law) in an attempt to capture the Gate, holding the Commonwealth hostage.
Colletta duly strikes, giving the other families the grounds to oppose him militarily. The revolt is put down, but at a price: the radio device and the Gateway are destroyed, and with it, the connection to our world. What little talent the Commonwealth has in physics works feverishly to re-establish the Gate. They are successful, but when they look through the new gate, they do not see FirstSide (New Virginia slang for Rolfe's home Earth) Oakland, but instead a snarling saber-toothed cat and a dead giant sloth.
Commonwealth of New Virginia
History
While trying to fix his radio in his basement, Rolfe accidentally created a gate to an alternate California in which Europeans had yet to discover the Americas. After looking at the new world on the other side of the Gate by himself, Rolfe brought a dozen trusted members of his wartime platoon to see for themselves. Later, while camping in an alternate California after hunting and panning for gold, the group agreed to keep the "Gate" a secret. They believed (correctly) that they could exploit the wealth it offered them, and they recognized Rolfe as the leader of this endeavor.
Using one of his men's ties to the Mafia to cover up where they were getting the gold, Rolfe and his allies gained the starting capital necessary to start their own country. To protect the secret of the Gate, they bought up the residential area in Oakland around the Gate, converting it into an industrial complex. Immigration to "New Virginia", as they called it, began with close family members and friends of the men involved, but it later included settlers drawn from various ethnic groups looking for places to hide: fugitive Nazis, pieds-noirs, Afrikaners, Rhodesians, and former Russian communists.