What major event marked the beginning of the Marine Corps' active duties in the early 20th century?

The event that signifies the beginning of the Marine Corps' active duties in the early 20th century is the involvement in the conflicts referred to as the Banana Wars, particularly in Nicaragua. This term encompasses a series of interventions throughout Central America and the Caribbean from the 1890s to the early 1930s. During this period, the Marine Corps expanded its role in military operations aimed at protecting American interests, stabilizing the region, and ensuring governmental control favorable to U.S. policies.

The significance of the Marine Corps during the Banana Wars cannot be overstated, as they developed tactics and strategies that would shape their future engagements. This era also marked a shift in the Marine Corps' focus from traditional European combat to supporting U.S. interests in Latin America, thereby solidifying their role as an agile and versatile force in various settings. The training and experiences gained during the interventions in Nicaragua specifically contributed to the overall expansion of Marine Corps capabilities leading into more significant global conflicts like World War I and World War II.