According to Howard Zinn, how and why did the United States take the Southwest from the Mexicans? What does Zinn say about how the war started? Why...

"He seems to have lost all respect for Mexican rights and is willing to be an instrument of Mr. Polk for pushing our boundary as far west as possible."  Colonel Ethan Alien Hitchcock

Howard Zinn retells the history of the Mexican-American War as a tale of deception and imperialism on the part of the United States government. After the Louisianna Purchase, large swaths of land in the southwest were part of an independent Mexico. The United States instigates a rebellion by Texas to declare independence from Mexico. The rebellion was successful and the United States quickly admitted the state to the Union. President James Polk is described as an expansionist at heart and despite civilian and military sentiment against a war with Mexico, he plows ahead with his imperial agenda. Polk would make it a habit of agitating Mexico. Soon after his inauguration he sent troops to the Rio Grande River, which was actually in Mexico, to enforce that river as the boundary between Texas and Mexico. Soon after this action, the Mexicans responded with the aggression that Congress needed to declare war on Mexico. The motives for the war are described by Zinn in the following excerpt:

Accompanying all this aggressiveness was the idea that the United States would be giving the blessings of liberty and democracy to more people. This was intermingled with ideas of racial superiority, longings for the beautiful lands of New Mexico and California, and thoughts of commercial enterprise across the Pacific.

The war was a striking success for the United States. With the American triumph, Mexico had lost about one-third of its territory, including nearly all of present-day California, Utah, Nevada, Arizona and New Mexico.