Complex sentence: Captain Hunter's orders were to establish a military alliance with the Pima natives in the area and to watch for a suspected Union advance from California which began with the capture of Fort Yuma and ended with the fall of Mesilla.
Split into simpler sentences: Captain Hunter's orders were to establish a military alliance with the Pima natives in the area and to watch for a suspected Union advance from California. The Californi invasion began with the capture of Fort Yuma and ended with the fall of Mesilla.