1845: Texas was admitted to the Union. This angered Mexico which still considered Texas to be its territory.
1846: A boundary dispute arose between the United States and Mexico. Both countries claimed an area of land between northern Mexico and southern Texas. The United States said the boundary line was the Rio Grande. The Mexicans claimed it was the Nueces River. The dispute led to the outbreak of the Mexican War.
1846-1848: General Zachary Taylor defeated Santa Ana at the Battle of Buena Vista. Colonel Stephen Kearney marched to California and helped American settlers overthrow the Mexicans in the Bear Flag Revolt. General Winfield Scott captured Veracruz and Mexico City, and Mexico surrendered.
1848: The United States won a large land area that today makes up the southwestern part of the United States. The United States paid Mexico $15 million for this area, known as the Mexican Cession.