The first half of the twentieth century brought the United States face to face with challenges unlike anything in its history. Two world wars reshaped global politics. The Great Depression brought the American economy to its knees. And through it all, a single President — Franklin Roosevelt — led the nation for longer than any other person before or since. This subchapter covers the period from 1914 to 1945, one of the most consequential periods in American and world history.

World War I (1914–1918)

In 1914, a major war broke out in Europe. At the time it was known simply as the Great War — it was only later, after a second global conflict, that it became known as World War I A global war fought primarily in Europe from 1914 to 1918, involving many countries on both sides. The U.S. entered the war in 1917 and fought until the war ended on November 11, 1918. . Countries from across the world joined the fighting on opposing sides.

The United States did not enter the war immediately. While the conflict began in 1914, the U.S. did not join until 1917 and fought until the war ended on November 11, 1918. Today, that date is commemorated as Veterans' Day A national U.S. holiday celebrated every November 11th, honoring all men and women — living and deceased — who have served in the U.S. military. The date marks the end of World War I in 1918. — a national holiday honoring all men and women, living and deceased, who have served in the U.S. military.

was President during World War I. He was the 28th President of the United States, serving from 1913 to 1921.

The Great Depression

When Franklin Roosevelt The 32nd President of the United States, serving from 1933 to 1945. He was the longest-serving President in U.S. history, elected four times, and led the country through both the Great Depression and World War II. was elected President in 1933, the United States was in the grip of the Great Depression The worst economic crisis in American history, characterized by widespread bank and business failures, mass unemployment, and extreme poverty. It began before Roosevelt took office and persisted through much of the 1930s. — the worst economic crisis in the country’s history. Banks and businesses failed across the country, employment collapsed, and millions of Americans lost both their jobs and their homes.

To understand why the Great Depression was so devastating, it helps to understand the American economic system. The United States operates a capitalist economy An economic system, also called a market economy, in which the government does not control the economy. People are free to start businesses and make money. This is the economic system of the United States. , also called a market economy. In a market economy, the government does not plan or control the economy, and individuals are free to start businesses and earn money. When that free-market system collapsed during the Depression, the consequences were felt by ordinary people everywhere.

Franklin Roosevelt: The Longest-Serving President

Franklin Roosevelt was elected the 32nd President of the United States in 1933. He went on to become the longest-serving President in U.S. history, elected to the presidency four times. He served from 1933 to 1945 and died while still in office in 1945.

Roosevelt’s extraordinary length of service led directly to a constitutional change. In 1951, the 22nd Amendment A constitutional amendment ratified in 1951 that limits the President of the United States to a maximum of two elected terms in office. It was passed following Franklin Roosevelt's four terms as President. was added to the Constitution. An amendment is a change to the Constitution. The 22nd Amendment established that no person can be elected President more than twice — a rule that applies to all Presidents today.

Roosevelt’s presidency spanned two of the most consequential periods in American history: he was President during both the Great Depression and World War II.

World War II (1939–1945)

In 1939, World War II began in Europe when Germany attacked Poland and then moved against other European nations. Japan and Italy aligned themselves with Germany, forming one side of the conflict. The United States did not enter the war at first.

The moment that drew America into the conflict came on December 7, 1941, when Japan launched a surprise attack on the U.S. naval base at Pearl Harbor A U.S. naval base in Hawaii attacked by Japan on December 7, 1941. The attack prompted the United States to enter World War II the following day. in Hawaii. On December 8, President Roosevelt announced that the United States would enter the war. Congress voted the same day to declare war — exercising one of the constitutional powers that belongs to the federal government. The U.S. fought in World War II from 1941 to 1945.

During World War II, the United States fought against Japan, Germany, and Italy. The U.S. was not alone: the United Kingdom, France, Russia, and other nations fought alongside the United States against those same countries. World War II ended in 1945 when both Germany and Japan surrendered.

Wrapping Up

The first half of the twentieth century tested the United States in ways no previous generation had experienced. World War I drew the country onto the world stage for the first time. The Great Depression shook American society to its foundations. And World War II — triggered by the attack on Pearl Harbor — made the United States a decisive force in global affairs. Franklin Roosevelt presided over both of the era’s great crises and became the longest-serving President in American history, directly prompting the two-term limit that the 22nd Amendment now enshrines. Every President, date, and event covered here can appear on the civics test.