Before World War II began in 1939, World War I (1914–18) was referred to as the Great War, and understandably so given its enormous impact. The price of the conflict was paid in human casualties: more than 10 million soldiers died and another 20 million were wounded. Civilian deaths were equally devastating, resulting from widespread hunger and flu epidemics (this was in the days preceding the advent of penicillin to treat complications from influenza).

During the 1920s, with the death toll of the Great War a recent memory, the civilized world enjoyed a period of relative peace. In the United States, the decade was known as the Roaring Twenties—ten years of prosperity and even frivolity, despite ratification of the Eighteenth Amendment to the U.S. Constitution (1919), which prohibited the sale and consumption of “intoxicating liquors.”

At the close of the Great War, the League of Nations had been established to handle disputes among countries and avoid another major conflict. This was followed in 1928 by the Kellogg-Briand Pact (also called the Pact of Paris), in which 15 nations agreed to settle conflicts by diplomacy rather than military might; eventually 62 nations ratified the agreement.

Meanwhile, in Germany an extreme sense of nationalism was taking hold: The Treaty of Versailles, which had ended the Great War, seriously weakened the nation, allowing the rise of the Nazi Party, led by Adolf Hitler (1889–1945). Nazis were determined to see their beloved homeland rise to power once again, and they found a ready following among the German people.

In 1929 a general downturn began in the world economy, triggered by the U.S. stock market crash of late October. As the United States fell into the severe and sustained economic crisis known as the Great Depression, other industrialized nations—including Germany—also felt the impact. Unemployment jumped to record levels in many countries, and a lack of social welfare programs resulted in the destitution of numerous families. Politicians and economists alike searched for solutions to the crisis, many turning to anti-free trade or isolationist policies to “protect their own.”

Meanwhile, postwar efforts to keep peace were proving ineffective: Japan invaded Manchuria (China) in 1931; Italy conquered Ethiopia in 1936; civil war raged in Spain (1936–39); India became the site of a bitter struggle between British rulers and Indian nationalists; and the Sino-Japanese War (which began in 1937 and would be absorbed by the outbreak of World War II) was fought in Asia. A dangerous alliance was forged when Germany, Italy, and Japan formed the Axis powers in 1936. Also, by 1933, Germany had become a totalitarian state known as the Third Reich. In 1938 Nazi armies took their first steps toward gaining supremacy when they marched into Austria and claimed it, setting the stage for World War II (1939–45).