During which conflict did the U.S. adopt the policy of containment?

The correct answer is that the U.S. adopted the policy of containment during the Cold War. This policy emerged in response to the growing influence and power of the Soviet Union following World War II. The main goal of containment was to prevent the spread of communism beyond where it already existed. This strategy was articulated in various documents, most notably in George F. Kennan’s "Long Telegram" and later the "X Article," which suggested that the United States should use a wide array of measures to counter Soviet expansionism, ranging from military involvement to economic aid and political alliances, such as NATO.

Containment significantly shaped U.S. foreign policy throughout the Cold War era, influencing decisions like the involvement in the Korean War and the Vietnam War, as well as actions that aimed to bolster non-communist governments around the world. The other conflicts listed—World War I, World War II, and the Spanish-American War—do not encapsulate the strategic approach of containment, as they were focused on different global dynamics and conflicts rather than the ideological struggle between the U.S. and the Soviet Union characteristic of the Cold War.