Final answer:
The build-up of the U.S. Navy in the late 19th century stemmed from an American attitudinal shift toward international involvement, championed by industrialists' search for foreign markets and Alfred Mahan's strategic military recommendations to protect economic interests and expand influence.
Step-by-step explanation:
The new American attitude toward international affairs which led to the building-up of the U.S. Navy was driven by industrialists' desires for access to foreign markets and the strategic military thinking influenced by Naval theorists like Alfred Mahan.
The belief that the country needed new markets to prevent economic downturns and the view that a strong navy was essential for protecting these economic interests and asserting international influence marked a significant shift away from 19th-century isolationism.
Mahan's seminal work, The Influence of Seapower upon History, recommended the expansion of the U.S. Navy, the establishment of a network of naval bases, and the construction of the Panama Canal to enhance the navy's operational range.
This perspective contributed to the Naval Act of 1890, initiating the construction of a modern steel fleet, signaling the U.S.'s move towards a more interventionist foreign policy and the rise of its naval power.