What does Howard Zinn say about how and why the United States took the Southwest (or northern lands of Mexico) from Mexico?

The Mexican War is the subject of Chapter Eight of A People's History of the United States, and Zinn's narrative of the conflict focuses on the intense opposition to the war among abolitionists and Whigs who opposed the expansion of slavery. These critics viewed the war as immoral, an unjustified act of aggression that many believed was aimed at the expansion of slavery into territories gained by the defeat of Mexico. Zinn makes it clear that he shares this perspective, and illustrates in detail the brutal nature of the American invasion of Mexico as well as its effects on the people who actually fought the war. A quote on page 164 sums it up: "It was a war of the American elite against the Mexican elite, each side exhorting, using, killing its own population as well as the other." This is Zinn's view of American conflicts in general, and the Mexican War is used to illustrate what he views as the immoral nature of war. It wrested a vast expanse of territory from a sovereign nation, and using the blood of working-class and poor Americans, it paved the way for territorial expansion that would ultimately tear the country apart. And like almost every American war according to Zinn, it was conducted without the support of many Americans.