Why did the Mexican-American War of 1846 occur?
A. Because the U.S. refused to acknowledge that it has won its independence from Spain
B. Because Mexico insisted that its border with the U.S. was at the Rio Grande
C. Because Mexico expected high reparations for the 1836 war with Texas
D. Because Mexico never recognized Texas' independence, and disputed its borders even after its annexation into the U.S.