Final answer:
John O'Sullivan's editorial refers to Texas, highlighting the significant American settlement and cultural impact in the region, resulting in its independence and playing a role in Manifest Destiny.
Step-by-step explanation:
The excerpt from the editorial by John O'Sullivan is referring to Texas. This is clear from the context of Anglo-Saxon emigration and the quest for dominion in territories north of the Rio Grande River. Texas was a Mexican province that saw a significant influx of American settlers, initially welcomed by the Mexican government but later leading to conflicts over cultural and legal differences, resulting in the Texas Revolution and eventual independence in 1836.
During the era of Manifest Destiny, American expansionists believed in the inevitable absorption of territories such as Texas into the United States. O'Sullivan's editorial captures the spirit of this belief, as settlers pushed westward bringing their customs, laws, and slavery, which was a contentious issue given Mexico's abolition of the practice. This movement was integral to the eventual shape of the nation and the balance of free and slave states prior to the Civil War.