Answer:
Mexico
Step-by-step explanation:
Following the conflict of the United States against Mexico that led to the Mexico-American war between 1842 to 1848.
The war resulted in the formation of the Treaty of Guadalupe Hidalgo. This treaty in turn led to the United States acquired more than 500,000 square miles of land. This increased the landmass of the United States by about one-third of its former territory.
Hence, in this case, the right answer is MEXICO.