Complex sentence: The syndicate operated from the Rocky Mountains to the Pacific, was most active in California and headquartered in San Francisco, active from about 1875 to 1898, and was at its peak from 1883 to 1886.
Split into simpler sentences: The syndicate operated from the Rocky Mountains to the Pacific, but was most active in California and was headquartered in San Francisco. It's tenure ran from about 1875 to 1898 and was at its peak from 1883 to 1886.