What is the main point of chapter 8 in A People's History of the United States?

Chapter 8 of
 portrays the American nation as one obsessed with a desire to expand its
territorial boundaries. The then-president of the United States, James K. Polk, was driven to
fulfill "manifest destiny" and used the disputed border region between Texas and
Mexico to help complete his mission.

Polk, a Democrat, provoked the Mexican
army to attack Americans along the Rio Grande region of Texas. There was virtually no doubt that
the United States would prevail in any conflict with Mexico. This victory over the U.S.'s
neighbors to the south would open the west to American expansion all the way to the Pacific
Ocean and the economic opportunities that body of water provided.

In
"We Take Nothing by Conquest, Thank God," Zinn claims the ensuing war was little more
than an immoral land grab done for the sole purpose of lining the pockets of wealthy elites.
There was little in the way of political opposition to Polk, even from the rival Whig Party.
A...