What event sparked the beginning of the Mexican War? ​

Mexico claimed the Nueces River as its northeastern border, while the U.S. claimed the Rio Grande River, and the day that both troops met at the Rio Grande and the Mexican army opened fire, on April 25, 1846, the Mexican American War began.

Explanation:

when Mexican troops cross north of the Rio Grande River and opened fire on U.S. troops at Fort Texas. Battle of Palo Alto

Explanation:

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the authority to tax.