The US Army, mercenaries hunters and settlers pushed westward in the 1870s and killed 31 million buffalo in 13 years.

D.A.
The mass slaughter of buffaloes in the 1870s was aimed at removing the main sustaining of the First Nations in the North American prairies. This animal extermination had a strong impact on the natives, both physically and spiritually.
As more settlers began to arrive in North America, conflicts arose between them and the native inhabitants.
The US government realized that if they killed the buffalo, Native Americans would lose their main source of livelihood.
The army was ordered to remove 20-25 million buffalo to defeat the mighty Lakota armies and their allies. The construction of railroads facilitated the killing. Buffalo hunters killed only for hides and left to rot the rest of the animal. During one winter (1872-1873) 1,500,000 buffalo hides.were exported to the East coast.

Railway companies offered trips westward to settlers just  to kill buffalo, even from the windows of the train, Competitions were held to hunt animals. During one of thjose, a person of Kansas established a record by killing 120 buffaloes in 40 minutes. Another man named "Buffalo" Bill Cody, who was hired to kill buffalo, in two years killed 4,000 animals. Due to different reasons an estimated total of 31 million buffaloes were killed between 1868 and 1881 and in 1885 only 500 buffalo remained (from an original population of 20-25 million). At the same time the native population in the region fell from 1 million to little more than 200,000.