World War I began in August 1914 with the German declaration of war against Russia and France. It ended four-and-a-half years later with the military defeat of the German army. The first modern war, World War I witnessed the use of new weapons that caused horrible injuries and mass destruction. Nearly two million German soldiers were killed, and two-and-one-half million returned from the front as “war cripples.” The so-called Great War was the first total war that involved countries' entire populations in the production and execution of war. This involvement also meant hardship, sometimes extreme, for noncombatants and demoralization on the home front.

The Vietnam War was never declared. It began as a conflict between France and the Vietnamese nationalist movement during World War II, when the Japanese temporarily displaced the French as colonial rulers and thus created an opening for the communist Viet Minh. The United States first sent military advisers to Vietnam in 1950 as part of its strategy to contain communism; troops followed later. The conflict in Southeast Asia took place thousands of miles from the United States, whose territory was never threatened. American civilians were not directly involved, whereas three million Vietnamese were killed. Approximately three million Americans served in the armed forces during the United States' direct involvement in Vietnam; fifty-eight thousand of them were killed. The longest war of the twentieth century ended in 1975 with the fall of Saigon, two years after the armistice of Paris had allowed the United States to withdraw its troops.