What were the consequences of the Mexican-American War for U.S. territory and politics?

• A. The U.S. gained Florida and Texas.
• B. The U.S. gained California and much of the Southwest, intensifying disputes over slavery in new territories.
• C. The war ended slavery nationwide.
• D. The war created the first transcontinental railroad.

Answer: (B)

The key idea is that the Mexican-American War dramatically expanded U.S. territory and sharpened national debates over slavery. After the war ended in 1848 with the Treaty of Guadalupe Hidalgo, the United States gained a huge swath of land—the Mexican Cession—including California and much of the Southwest (areas that would become California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming). This sudden territorial growth raised the question: should slavery be legal in these new areas? That tension over whether slavery would expand into new lands consumed national politics, helping drive the Compromise of 1850, the Fugitive Slave Act, and the realignment of political parties around the issue. Other choices don’t fit because Florida had been acquired earlier, Texas’s later annexation is separate from this war, the war did not end slavery nationwide, and the first transcontinental railroad was built decades later through different events.