Final answer:
Manifest destiny, coupled with the Mexican-American War, served as a key basis for US foreign policy, supporting the belief in America's divinely sanctioned expansion and leading to significant territorial gains like California.
Step-by-step explanation:
Manifest destiny and the Mexican-American War were accepted as the basis for the US foreign policy during much of the 19th and 20th centuries. The philosophy of Manifest Destiny held that it was the destiny and duty of the United States to expand westward across North America, spreading its democratic institutions. This belief justified American territorial expansion as inevitable, just, and divinely ordained.
The drive for expansion played a significant role in the annexation of territories such as Texas and Oregon and was a primary cause of the war with Mexico, which led to the acquisition of additional lands in the Southwestern United States including California.
The concept of Manifest Destiny provided a religious and moral justification for the American expansion that reshaped the nation and its foreign relations. While it was widely supported, it also exacerbated the divide between free and slave states and complicated the balance maintained by the Missouri Compromise. Ultimately, the era of Manifest Destiny left a profound impact on the United States' relationships with other nations, especially neighboring Mexico and European powers that watched the growing military strength of the United States with a wary eye.