What was the one thing responsible for pushing the mountain men into the west?

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By the time two new international treaties in early 1846 and early 1848[1] officially settled new western coastal territories in the United States and spurred a large upsurge in migration, the days of mountain men making a good living by fur trapping had largely ended. This was partly because the fur industry was failing due to reduced demand and over trapping. With the rise of the silk trade and quick collapse of the North American beaver-based fur trade in the later 1830s–1840s, many of the mountain men settled into jobs as Army Scouts or wagon train guides or settled throughout the lands which they had helped open up.

a phrase from a poem by ralph waldo emerson about the battle of lexington and concord. emerson's words read, “here once the embattled farmers stood / and fired the shot heard round the world.”