In 1846, a war broke out between two neighboring countries. The United States and Mexico went to war over land that they both claimed to own. President Polk spoke to the U.S. Congress after Mexican troops attacked and killed eleven Americans north of the Rio Grande. He said that the United States had to protect itself from aggression. Americans quickly moved to claim land, for example, in California where American settlers, including “Kit” Carson, created a flag showing the California grizzly. These settlers hoisted the California flag into the air and claimed territory for the United States.

President Polk appointed General Zachary Taylor to lead the U.S. army against the Mexican army. President Polk did not understand the terrain on which the American troops had to fight and assumed they could gather the food and water they needed from the land. In reality, the troops were in the desert and needed constant supplies. The president made another mistake when he decided to negotiate with the former dictator of Mexico, Santa Anna. Santa Anna had somehow gotten word to the president that in return for his escape from exile in Cuba, he could negotiate the end of the war in favor of the United States. Instead, however, when Santa Anna returned to Mexico, he took control of the army and began fighting against U.S. troops.

In the midst of this adversity, the troops fought bravely. In September 1847, U.S. troops entered Mexico City. In 1848, the Treaty of Guadalupe Hidalgo was signed. The treaty gave the United States all land claims to Texas north of the Rio Grande as well as California and New Mexico. The United States agreed to pay Mexico $15 million.