The document discusses three key factors that opened up the American West to migration in the late 19th century: 1) The end of the Indian Wars in 1885 marked by Geronimo's surrender, removing the threat to settlers' safety; 2) The invention of barbed wire in 1874, which allowed for fencing of the Great Plains and ended the era of open cattle drives; 3) The expansion of the railroad network throughout the West in the late 1800s, which provided access to land and resources and encouraged further settlement.