Final answer:
John O’Sullivan argued for U.S. expansionism based on Manifest Destiny, which posited that the country was destined and morally justified to spread democracy and its culture across North America, leading to the negotiation of the 49th parallel with Great Britain, among other territorial acquisitions.
Step-by-step explanation:
Manifest Destiny was the doctrine that became the rallying cry for U.S. territorial expansion in the 19th century. Coined by newspaper editor John O’Sullivan, the term embodied the belief that Americans were divinely ordained to expand democratic institutions across the continent. This belief system provided the moral and political rationale for the annexation of vast tracts of land.
During President James K. Polk's administration, the Manifest Destiny ideology was at its peak. Polk, an expansionist, catered to the American public’s appetite for more territory, particularly the Oregon Territory, where the U.S. and Great Britain had competing claims. The acquisition of territory was not only seen as America’s divine right but also as a means to advance Protestant, democratic values and bring the perceived benefits of a "superior" culture to the native inhabitants and new territories.
Ultimately, this expansionist ideology influenced negotiations with Great Britain, leading to the establishment of the 49th parallel as the boundary. O’Sullivan's arguments and the widespread support for Manifest Destiny aided the United States in justifying its territorial acquisitions, including Oregon, Texas, and following a provoked conflict, territories from Mexico - leading to a nation that stretched from the Atlantic to the Pacific.
However, these expansionist actions raised numerous ethical and political debates over morality, military and economic security, and the balance between free and slaveholding states in the Union. The expansion was undertaken with a clear preference for white American settlers, as it often would spread slavery and displace Native American tribes.