Which event marked a significant increase in U.S. involvement in Asian affairs?

The Spanish-American War marked a significant increase in U.S. involvement in Asian affairs primarily because it resulted in the United States acquiring territories in the Pacific, particularly the Philippines. The war, fought in 1898, stemmed from the U.S. intervention in Cuba's struggle for independence from Spain. Following the victory, the Treaty of Paris not only concluded the war but also ceded the Philippines, Guam, and Puerto Rico to the United States.

This territorial acquisition positioned the U.S. as a colonial power in Asia and initiated a period of American imperialism that greatly influenced U.S. foreign policy in the region. The belief in America's role as a "civilizing force" led to further involvement in Asian affairs, including the suppression of the Philippine-American War, which followed the acquisition of the Philippines. Hence, the outcomes of the Spanish-American War fundamentally changed the dynamics of U.S. involvement in Asia and marked the beginning of a new era of American expansionism beyond its continental borders.