The west began to take in a larger population between the 1840's and the 1890's. There were many factors which contributed to this increase in the west some of which were the natural environment, the government, railways, and the migration of the oppressed Mormons along with poor people wanting cheap land. Which of these was the most significant factor? I would have to say the natural environment. Many cities were formed around gold mines, rivers, and the areas yet to be inhabited by the easterners.
The government played a role in getting people to the west. They wanted Texas to join the union very badly and they also fought the British hard for Oregon. By 1945 only a small portion of Oregon was in control by the government and Texas was practically part of the union. The government sold land on their Oregon claim very cheap and there were many more Americans out west in Oregon than British.
The British not wanting to waste man power in a place where they had only mustered around 3,000 civilians living there, decided to sell it to the United States for a pretty penny.
Not very many people wanted to live in the desolate west. That is why it was a perfect place for the oppressed Mormons to move to. Over a few years thousands of Mormons made the trek to Salt Lake City in the what is now Utah. The western population greatly grew because of the migration of the Mormon Pioneers but to call it the most significant would not be the most correct of statements.
The railroads made it a lot easier to come west. In the 1850's the Pacific Railroad promoted going west. The Gadsden Purchase enabled the South to claim the coveted railroad with much more ease. The...