The war with Mexico changed our nation significantly, however, your text and Zinn's People's History have different interpretations concerning this turning point.
Zinn's main points are that from the beginning of his presidency, Polk was determined to acquire California as part of his plan for expanding the continental boundaries of the United States. He also states that the incident leading to the “war on Mexico” was largely manufactured by the powerful advocates of Manifest Destiny and that attitudes against the war on Mexico were widespread, particularly among abolitionists. Zinn proposes that after the Mexican government prohibited immigration into Texas, large numbers of American settlers illegally immigrated into Mexican territory and once settled, Texan colonists failed to abide by the national laws and customs of Mexico. He claims that despite the years between battles and the different issues that bring us into conflict, the language of those who favor war often shows strong continuity. Zinn argues that the war with Mexico was fought by ordinary Americans and Mexicans but was driven by the interests of elites from both countries and that the early foundations of the United States tradition of civil disobedience have their roots in the war with Mexico. Finally, he claims that the war with Mexico was fueled by racist conceptions of Mexicans as inferior and a less “civilized” people.
Your text takes a different position. It claims American expansionism gained momentum in the 1840s, leading first to the acquisition of Texas and Oregon, and then to the Mexican War, which added vast southwestern territories to the United States and ignited the slavery question. The text also proposes that American international prestige grows as the United States expands. Successful military campaigns against Mexico along with well negotiated treaties with Britain force Europe to respect America more while Latin America begins to be wary of the "Colossus of the North." The text states that American forces quickly conquered California and New Mexico. Winfield Scott’s and Zachary Taylor’s invasion of Mexico was also successful, and the United States obtained large new territories in the peace treaty. It also claims that besides adding California, New Mexico, and Utah to American territory, the Mexican War trained a new generation of military leaders and aroused long-term Latin American resentment of the United States. Most important, it forced the slavery controversy to the center of national debate, as first indicated by the Wilmot Proviso.
