Which major conflict did the United States engage in during the 1900s?

The major conflict that the United States engaged in during the 1900s that is most recognized is World War I. This war, which lasted from 1914 to 1918, saw the U.S. enter the conflict in 1917, joining the Allies against the Central Powers. The involvement of the United States marked a significant shift in the war, contributing to the eventual defeat of the Central Powers and leading to the establishment of the League of Nations, although the U.S. did not join the League. The societal, political, and economic impacts of World War I were profound and shaped the trajectory of U.S. foreign and domestic policy in the following decades.

The Civil War occurred earlier, from 1861 to 1865, focusing on issues such as states' rights and slavery. The Spanish-American War, fought in 1898, was a conflict primarily over Cuba's independence but did not occur in the heart of the 1900s. The War of 1812 happened in the early 19th century, primarily about maritime rights and nationalism. Each of these wars is significant in American history, but World War I is the major conflict most directly aligned with the timeline provided in the question.