What was the incident between U. S. forces and Mexican forces called that caused the U. S. to declare war on Mexico?

On May 9, 1846, Polk began to prepare a war message to Congress, justifying hostilities on the grounds of Mexican refusal to pay U.S. claims and refusal to negotiate with Slidell. That evening he received word that Mexican troops had crossed the Rio Grande on April 25 and attacked Taylor’s troops, killing or injuring 16 of them. In his quickly revised war message—delivered to Congress on May 11—Polk claimed that Mexico had “invaded our territory and shed American blood on American soil.”

Explanation:

Mexican refusal to pay U.S. claims and refusal to negotiate with Slidell.

Mexican troops had crossed the Rio Grande on April 25 and attacked Taylor’s troops, killing or injuring 16 of them.

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