Final answer:
During the Battle of Palo Alto, Major Samuel Ringgold became one of the initial American casualties of the Mexican-American War.
Step-by-step explanation:
The significant event that occurred during the Battle of Palo Alto on May 8-9, 1846, was that Major Samuel Ringgold became one of the first American casualties of the Mexican-American War. He was struck by cannon fire, which led to fatal injuries. His death made him an early martyr for the American cause.