Final answer:
President James K. Polk expanded the U.S. by annexing Texas, negotiating over Oregon, and waging the Mexican-American War, all of which exemplified the era's Manifest Destiny beliefs and the country's desire for a transcontinental republic.
Step-by-step explanation:
President James K. Polk and the rest of America 'got lucky' through a series of territorial expansions that took place during Polk's presidency in the 1840s, a period when the notion of Manifest Destiny was at its peak. The annexation of Texas and the acquisition of Oregon Territory were major campaign promises that Polk fulfilled, expanding American territory significantly. The country's frontier stretched dramatically from the Atlantic to the Pacific, encompassing wide swaths of land that were considered vital for the spread of democracy and Protestant values.
Polk's administration navigated through international disputes and domestic pressures, exemplified by the diplomatic settlement with Britain over Oregon, drawing the boundary at the 49th parallel and securing Puget Sound. The U.S. readied for a potential conflict with Mexico over western lands such as New Mexico and California, which were also coveted. Polk's willingness to resort to military strength was demonstrated when he engaged the country in the Mexican-American War, a controversial decision that led to America's victory, further territorial expansion, and domestic debate over the spread of slavery.
The eagerness to expand territory, especially to the Pacific shores, and to consolidate a coast-to-coast republic, was partly attributable to geopolitical strategy, such as preventing a British-Texan alliance. The subsequent acquisition of lands after the war with Mexico showcased the contrast in treatment between Great Britain—a powerful adversary that necessitated diplomacy—and Mexico, perceived as weaker and thus subjected to military force by Polk's administration.