The U.S. has been heavily engaged in foreign affairs and international peacekeeping since WWII. In the 19th century most of our international conflicts were in response to piracy, or were attempts of imperialism. Were there any incidents where we intervened in foreign affairs for the diplomatic reasons of peacekeeping we so often see today? Could our 19th century global policy be considered isolationist, and if so, did it benefit or hinder the prosperity of our nation?
The sort of intervention you are talking about did not really begin until the 1898 Spanish-American War. However, the groundwork for foreign intervention of this sort (seizing colonies in the Philippines, Puerto Rico, and Guam or protectorates in Cuba) was a continuation of US policies like Manifest Destiny or Secretary of State Seward's purchase of Alaska or Pulitzer and Hearst encouraging US interventionism.
Manifest Destiny encouraged the spread of WASP American exceptionalism through the then-frontier of the Western United States which were the original foreign colonies of the Eastern seaboard.
Turner's thesis which was released in the late 1800s provided an intellectual framework for conceptualizing US expansion. It argued that the existence of frontiers had led to the technological and geopolitical advances of the United States. This conceptual framework encouraged the United States to pursue more expansionist policies that intervened more in other parts of the world.
McKinley's Secretary of State John Hays would advocate for the Open Door in China during the turn of the century. Then Theodore Roosevelt would continue these realist policies with his actions to seize the Canal Zone and build the Panama Canal.
In short, although the spread of US power began around the turn of the century it had earlier ideological and cultural roots.
Sources:
The American Age by Walter LaFeber