Final answer:
The most significant long-term result of the Mexican American War was the exacerbation of the slavery debate, which contributed to the Civil War. It also fostered Latin American distrust and the expansion of American territory, which upset the balance between slave and free states.
Step-by-step explanation:
The most important long-term result of the Mexican-American War was the intensification of the slavery debate, which eventually contributed to the sectional crisis leading to the Civil War. The war drew the United States into foreign conflicts, creating lasting distrust in Latin America, particularly due to perceived American aggression and expansionism often referred to as "Yankeephobia". The acquisition of a vast amount of territory through the Treaty of Guadalupe Hidalgo stirred the question of whether these new territories would enter the Union as slave states or free states, upsetting the balance of power between North and South.
The Wilmot Proviso, although it never became law, evidenced the strong opposition of the North to the spread of slavery into the newly acquired lands. In conclusion, while the war solidified the United States' presence as a continental power, it also exacerbated the national divide over slavery that loomed over its future.