Final answer:
The Mexican American War was a significant military success for the U.S., resulting in extensive territorial gains, but it was also costly in terms of lives and money and had lasting political and international consequences.
Step-by-step explanation:
The Mexican-American War was indeed a huge military success for the United States, characterized by significant territorial gains and the demonstration of military prowess. The U.S. Army's occupation of Mexico City and its success in extending the nation’s territory to the Pacific Ocean exemplify this victory. The Treaty of Guadalupe Hidalgo, which concluded the conflict, resulted in the United States acquiring a vast expanse of land that became states such as California, Arizona, New Mexico, and others, fueling the concept of Manifest Destiny.
However, the war also had substantial costs. Approximately 13,000 American soldiers died, predominately from disease, and the conflict cost around $100,000,000. Beyond the financial and human toll, the war had enduring ramifications on U.S. domestic politics, particularly the slavery debate, and on international relations, triggering 'Yankeephobia' across Latin America.
Ultimately, the war raised the critical issue of slavery’s future in the new territories, resulting in increased tensions that contributed to the sectional crisis preceding the American Civil War.