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

World War II is the correct answer because it was a significant global conflict that involved the United States from 1941 to 1945. The U.S. entered the war following the attack on Pearl Harbor and played a pivotal role in the defeat of the Axis powers, which included Germany, Italy, and Japan. This conflict had profound impacts on the world and American society, leading to significant changes in international relations and the establishment of the United States as a superpower.

The other conflicts mentioned occurred in different centuries: The American Revolution took place in the late 18th century, fighting for independence from British rule. The War of 1812 also occurred in the early 19th century, primarily involving issues related to trade and territorial expansion. The Civil War was fought in the 1860s, primarily over issues surrounding slavery and states' rights. Each of these conflicts shaped the United States in various ways, but they are not part of the major conflicts in the 20th century, such as World War II.