Time's Eye by Arthur C. Clarke

After a mysterious “Discontinuity” shatters Earth into a patchwork of eras, 21st‑century UN personnel, stranded astronauts, 19th‑century British soldiers, and figures like Alexander the Great and Genghis Khan collide on a hybrid world. As alliances form and battles rage, the survivors seek the meaning behind hovering silver “Eyes” that observe and intervene, revealing hints of a powerful intelligence manipulating humanity’s past and future.