The turning point of the American-Mexico war could be seen during the Battle of Monterrey and Buena Vista. General Taylor from the American side was able to occupy the city of Matamoros and Camargo after major American success in California, the Pacific Coast and parts of Northeastern Mexico. However Taylor met considerable challenges when he tried to occupy Monterrey and at the first attempt massive casualties occurred on both sides. Taylor’s infantry units were repulsed by General Ampudia of the Mexican side. Urban warfare was new to the American soldiers who walked straight into an ambush from Mexican fighters. This forced Taylor to change tact after being advised by Texan soldiers who understood urban warfare better having experience with the Mexican style in prior conflicts. These new techniques changed the tide for the Americans who pushed and eventually trapped the Mexican fighters. Taylor was able to reach Buena Vista and there he personally engaged with Santa Anna’s massive army but who later retreated to attend to political issues in Mexico City leaving Taylor and his forces in control of the Northeastern part of Mexico.
There is no one battle that is typically identified as the turning point of this war. The reason for this is that a turning point is typically a point at which one country that had been losing the war turns things around and starts to win. The US was never in any serious danger of losing the war so it is hard to identify a turning point battle.
The battle that most closely qualifies was the Battle of Buena Vista in Coahuila state in northern Mexico. This battle was part of the US invasion of northern Mexico. Before the battle, the US troops had been moving slowly. They had gotten bogged down in urban fighting in the city of Monterrey. After that, they moved on and fought another major battle at Buena Vista. This was another difficult battle. The fighting was somewhat indecisive, but Santa Anna withdrew, largely because he had gotten word of political unrest in Mexico City. Once he withdrew, northern Mexico was open to the US forces. From there on, the war was relatively easy, making the Battle of Buena Vista something of a turning point in the war.