The United States gained the most territory from Mexico through the Mexican Cession in 1848 and obtained the Oregon Country from Great Britain in 1846.
After the Louisiana Purchase, the United States gained the greatest amount of territory from Mexico through the Mexican Cession in 1848. This acquisition included California, Arizona, New Mexico, Nevada, Colorado, Utah, and parts of Wyoming.
The region the United States obtained from Great Britain was the Oregon Country, which came into the Union in 1846 through the Oregon Compromise.
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