Final answer:
During the long 19th century, U.S. foreign policy was most significantly influenced by expansionism, as America shifted from isolationism to actively pursuing imperialism, with the Monroe Doctrine and growing nationalism also shaping its path to increased global power.
Step-by-step explanation:
U.S. Foreign Policy Evolution in the Long 19th Century
During the long 19th century, which spans from 1776 to 1914, the most significant influence on the evolution of U.S. foreign policy was likely expansionism. This period witnessed the United States' transformation from a nation adhering to isolationist principles toward one with an aggressive policy of territorial expansion, influenced by both the philosophy of Manifest Destiny and a response to European imperialistic activity.
Driven by these ideas, the U.S pursued a policy of imperialism, extending its political, economic, and military control over other nations.
The Monroe Doctrine, issued in 1823, was a substantial foreign policy doctrine that reflected growing American nationalism and the desire to prevent further European colonization in the Americas, which was an offshoot of the expansionist mindset.
Nonetheless, the overall trend toward increased global power, characterized by engaging in international affairs and building an empire, was underscored by the late 19th century's era of imperialism and America's participation in global economic and political relations.
As the United States entered the final decades of the 19th century, the country was inspired by the imperialistic actions of European powers and sought to expand its own territory and influence, which had profound and lasting effects on its foreign policy and global standing, defining America’s march towards becoming a world power before World War I.