After a costly four-year war the United States won the Mexican American War annexing all of Mexico. This doubled the USA's size, accelerating the United States OTL growth into a world power.
Point Of Divergence
Santa Anna took a wrong turn on his return to Mexico from the US causing him to accidentally go to a military base, after seeing the intimidating forces at this base he decided, upon his return to Mexican Power, to draft extra soldiers for the war effort.
The Mexican forces under General Mariano Arista prepared for war. On April 25, 1846, a 2000-strong Mexican cavalry detachment attacked a 70-man U.S. patrol under the command of Captain Seth Thornton, which had been sent into the contested territory north of the Rio Grande and south of the Nueces River. In what would come to be called the Thornton Affair, the Mexican cavalry attacked the patrol, killing 16 American soldiers
With this obvious act of war being committed on what they believed to be their land the United States Congress voted to invade Mexico with only 14 Whigs voting no for fear of the South using the war to further their pro-slavery agenda. Regardless the war moved forward as The US prepared their soldiers for a Mexican invasion.
After promising to advocate for peace in return for passage home Santa Anna has his experience with the US military and, upon achieving power, drove for sweeping drafts. The Mexican army doubled in size by the end of the year and recruits came into service daily until late 1949 when most of Mexico had fallen to the US.
Conclusion to Mexican-American War
The war lasted until to September 30 of 1850 when the Mexican forces surrendered their last stronghold in Oaxaca after a 37-day long siege by American troops. The United States quickly occupied the area forcing each state to write a Congress-approved constitution and making them American territories.