B. A fort was established and many were killed by disease.Fort Clark was soon established at the Mandan villages. At first it provided the Mandans with a useful alternative to trading with the British and also offered military support from their traditional native enemies, the Sioux. However, Americans at the fort unwittingly brought new diseases to the area that decimated the local native population. Where the Mandans had a thriving and sophisticated trading center when Lewis and Clark arrived in 1804, by the late 1830s their total population had been reduced to less than 150. The nation’s growth combined tragedy and triumph at every turn.