Mostly due to the ideas of Manifest Destiny. They started going westwards with the desire to span the United States from coast to coast, and then they started going worldwide. The idea behind it is that the United States were destined to help the world become free and democratic like they were so they started helping people worldwide in rebellions against colonialism or in similar affairs.
<span>The factory system brought people from the farms to the cities to work together.
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Manifest Destiny is a term for the attitude prevalent during the 19th century period of American expansion that the United States not only could, but was destined to, stretch from coast to coast. This attitude helped fuel western settlement, Native American removal and war with Mexico.
Yes, the Mexican American residents of these territories did become U.S. citizens.
