Hell's Gate (novel)
Sharona and Arcana are worlds in the multiverse which hold human civilizations. Both have discovered portals, which allow one to pass from one Earth to another, parallel Earth, which is almost always similar but uninhabited by sentient beings. Chains of universes linked by portals are called transit chains. Most universes have just one portal, or two; a few have three, and both civilizations have only found one with four.
While the Sharonans developed technology approximately equivalent to industrialized nations at the beginning of the 20th century, they also have psionic "Talents", which allow Talented individuals to communicate mentally, view events happening at great distance, learn the histories of objects simply by holding them, et cetera. The Arcanans are capable of magic, which they use as a substitute for advanced technology (although their straightforward technology is medieval, they possess what are essentially magic-based computer systems).
The story begins with a Sharonan company's survey crew exploring a new universe, which they discover contains eight portals in close proximity. One of the members of the crew goes to hunt by himself, and has a surprise encounter with an Arcanan, also from a survey crew exploring the universe (although his crew is part of a governmental military organization). Each one is afraid that the other will shoot, so they raise their weapons, and then one panics and fires (who fired first was never made clear). The Sharonian managed to stagger back to his crew and warn them before expiring; they immediately began to move back toward the portal they had come from.
Meanwhile, the Arcanan unit's commander, Jasak Olderhan, upon finding his dead soldier, decided to pursue the Sharonans and attempt to make peaceful contact before they could get back to their portal and report what had happened. They caught up with the Sharonan survey crew; one of them stood up with his hands raised in an attempt to show peaceful intentions. Despite orders from the Arcanan commander not to do so, an Arcanan NCO fired his arbalest at the Sharonan and killed him. The Sharonans returned fire, killing the NCO, and battle broke out. The Arcanans were victorious but suffered heavy casualties.
The Arcanans withdrew to the universe they had been in before encountering the Sharonans, taking the few surviving Sharonan survey team members with them. However, one of the Talented individuals managed to send a telepathic message to another "Voice" in the moments before she was incapacitated, alerting Sharona to the situation. Sharonan soldiers pursued the Arcanans across the eight-portal universe, and a representative demanded the return of their survey crew members. The new Arcanan commander, who had replaced Olderhan, did not realize that the Sharonans knew what had happened, and attempted to capture the representative. The Sharonans began to fire artillery through the portal, and because magic does not work through a portal, the Arcanans were unable to return fire. The Arcanan forces in the neighboring universe were essentially destroyed, with only a few avoiding death or capture.
The story goes on to discuss the situation on Sharona, where various countries are attempting to form a world government to handle the crisis.