America, once focused internally, found itself increasingly drawn into global conflicts in the 20th century, profoundly shaping its identity and role in the world. After two devastating World Wars, the nation transitioned into a decades-long ideological showdown known as the Cold War.

The World Wars: A Superpower Emerges

World War I (1914-1918)

Initially, the U.S. tried to remain isolationist as war raged in Europe. However, German submarine warfare (like the sinking of the Lusitania) and the Zimmermann Telegram (a German proposal for Mexico to ally against the U.S.) eventually pushed America into the conflict in 1917.

• Impact on the Home Front: The war galvanized American industry and society. Women stepped into factory jobs, and African Americans migrated north for industrial work (the Great Migration).
• Global Role: President Woodrow Wilson pushed for a "peace without victory" and proposed the League of Nations, an international body to prevent future wars. Though the U.S. never joined the League, it signaled a new, if hesitant, American engagement in global affairs.

World War II (1939-1945)

The rise of fascism in Europe and Asia (Nazi Germany, Imperial Japan) eventually shattered America's renewed isolationism. After the attack on Pearl Harbor in December 1941, the U.S. entered WWII.

• Home Front Transformation: The war effort was massive. Factories churned out planes, tanks, and ships. Rationing, war bonds, and Rosie the Riveter (symbol of women in wartime industry) became common. Japanese Americans were tragically interned in camps due to baseless fears.
• Global Leadership: The U.S. became the "arsenal of democracy," supplying allies and ultimately playing a decisive role in defeating the Axis powers. The development of the atomic bomb (Manhattan Project) and its use on Hiroshima and Nagasaki brought the war to a swift, devastating end and ushered in the nuclear age.
• Post-War Order: The U.S. emerged as the world's leading economic and military power, instrumental in creating international institutions like the United Nations and the World Bank.

The Cold War: An Ideological Showdown (1947-1991)

No sooner had WWII ended than a new conflict began: the Cold War, an intense rivalry between the United States (and its democratic, capitalist allies) and the Soviet Union (and its communist bloc). It was "cold" because it rarely involved direct military conflict between the two superpowers but was characterized by:

• Ideological Clash: Democracy vs. Communism, Capitalism vs. Command Economy.
• Containment: The core U.S. foreign policy was to "contain" the spread of communism globally. This led to:
  • Truman Doctrine (1947): Pledging U.S. support to free peoples resisting communism.
  • Marshall Plan (1948): Massive economic aid to rebuild war-torn Western Europe, preventing communist takeovers.
  • NATO (1949): A military alliance formed to counter Soviet expansion in Europe.
• Arms Race: Both sides rapidly developed and stockpiled nuclear weapons, leading to a terrifying balance of power (Mutually Assured Destruction - MAD).
• Proxy Wars & Interventions: The U.S. and USSR supported opposing sides in conflicts around the world (e.g., Korean War, Vietnam War, Cuban Missile Crisis, interventions in Latin America and the Middle East).
• Space Race: A symbolic competition for technological and ideological superiority.

The Cold War defined American foreign policy and much of its domestic life for over four decades, leading to intense fear (e.g., McCarthyism), massive defense spending, and a permanent shift in America's role from regional power to global superpower. Its end in 1991, with the collapse of the Soviet Union, marked the beginning of a new, complex era.