Answer:
Much of our expansion in the 1800's was conquest and settlement rather than colonization. Manifest Destiny and our westward movement systematically depopulated Native lands in favor of permanent white settlements. This was similar to earlier British policy of colonization in the 1700's, but different from the actions of empires in the later 1800s
Or
The United States enjoyed a period of rapid growth during the nineteenth century. In terms of territory, the United States more than doubled in size with the Louisiana Purchase. The young nation then gained the Southwest and California in a controversial yet successful war against Mexico. During the early 1800's, settlers moved westward over the Appalachian Mountains into the new states and territories. Many of these pioneers even settled beyond the country's western boundary. They flocked into Texas, California, and other western lands belonging to Mexico.
Or
During the mid-1800's, the United States gained control of the Mexican lands and the southern part of the Oregon Country, and the nation extended from coast to coast. The build-up of the West gave rise to changes in American politics. As areas in the West gained large populations, they were admitted to the Union as states.
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