The United States in the World Wars (1914–1945)
In the thirty-one years between 1914 and 1945 the United States moved from continental seclusion to global dominance. The two world wars accelerated industrial modernisation, federal expansion, the civil-rights agenda and an internationalist foreign policy that would shape the post-1945 order.
The doctrine, dominant in American foreign policy between Washington's 1796 Farewell Address and Pearl Harbor, that the United States should avoid entanglement in European political and military alliances. The Senate's rejection of the Treaty of Versailles in 1919 and the Neutrality Acts of 1935–37 expressed its interwar form.
The First World War (1914–1918)
President Woodrow Wilson declared neutrality on 4 August 1914 and was re-elected in 1916 on the slogan "He kept us out of war." Three factors eroded American neutrality:
- Allied trade — by 1917 the US exported nearly $4 billion in goods and credit to Britain and France.
- German submarine warfare — the sinking of RMS Lusitania on 7 May 1915 killed 1,198 people including 128 Americans. Germany's resumption of unrestricted submarine warfare on 1 February 1917 broke the Sussex Pledge.
- The Zimmermann Telegram (Jan 1917, made public 1 March) — Germany proposed alliance with Mexico against the US.
Wilson asked Congress for war on 2 April 1917; the declaration was approved on 6 April 1917.
American mobilisation
| Measure | Year |
|---|---|
| Selective Service Act | 18 May 1917 |
| Espionage Act | 15 June 1917 |
| Sedition Act | 16 May 1918 |
| War Industries Board (Bernard Baruch) | 1917 |
| Food Administration (Herbert Hoover) | 1917 |
Some 4 million men were drafted; about 2 million served in the American Expeditionary Forces (AEF) in Europe under General John J. Pershing. AEF actions at Belleau Wood (June 1918), Saint-Mihiel (September) and the Meuse-Argonne Offensive (Sept–Nov 1918) helped break the German lines. US war dead totalled about 116,000.
The peace and the rejection
In January 1918 Wilson outlined the Fourteen Points — freedom of the seas, open diplomacy, self-determination, and a League of Nations. He led the American delegation to the Paris Peace Conference (January 1919), signing the Treaty of Versailles (28 June 1919). But the US Senate, led by Republican Henry Cabot Lodge, rejected the Treaty on 19 November 1919 (49–35) and again on 19 March 1920. The United States never joined the League of Nations.
The interwar decades (1920–1941)
"Roaring Twenties"
Republican presidents — Warren Harding (1921–23), Calvin Coolidge (1923–29), Herbert Hoover (1929–33) — favoured low taxes, high tariffs (Fordney-McCumber 1922, Smoot-Hawley 1930), and immigration restriction (1921, National Origins Act 1924). Cultural change accelerated: Prohibition (Eighteenth Amendment, 1920–33), Women's suffrage (Nineteenth Amendment, 18 August 1920), the Harlem Renaissance of Langston Hughes and Duke Ellington, Charles Lindbergh's solo Atlantic flight (20–21 May 1927), and the rise of mass automobile culture (Henry Ford's Model T topped 15 million units by 1927).
The Great Depression
The Wall Street Crash of 24–29 October 1929 ("Black Thursday/Tuesday") wiped out roughly $30 billion in three weeks. By 1933:
- GDP had fallen by about 30%.
- Unemployment reached 25% (about 15 million workers).
- Industrial production had fallen by 46%.
- 9,000 banks had failed.
President Hoover's response — Reconstruction Finance Corporation, Hoover Dam, the protectionist Smoot-Hawley tariff — was insufficient. Franklin Delano Roosevelt won the 1932 election with 472 to 59 electoral votes.
The New Deal (1933–1939)
FDR's First Hundred Days produced a torrent of legislation:
- Emergency Banking Act (9 Mar 1933).
- Civilian Conservation Corps (CCC).
- Agricultural Adjustment Act (AAA) and National Industrial Recovery Act (NIRA).
- Tennessee Valley Authority (TVA) — the era's signature regional development project.
- Federal Deposit Insurance Corporation (FDIC) — banking deposit insurance.
- Glass-Steagall Act — separated commercial and investment banking.
The Second New Deal (1935) added the Social Security Act (14 August 1935), the Wagner Act protecting unions, and the Works Progress Administration (WPA). FDR was re-elected in 1936 with 60.8% of the popular vote.
- 6 April 1917 — US declares war on Germany.
- 11 November 1918 — Armistice; US war dead ~116,000.
- 19 November 1919 — Senate rejects Treaty of Versailles.
- 18 August 1920 — Nineteenth Amendment ratified.
- 24 October 1929 — Black Thursday; market crashes.
- 4 March 1933 — FDR inaugurated; "nothing to fear but fear itself".
- 14 August 1935 — Social Security Act.
- 7 December 1941 — Pearl Harbor; US enters WWII.
- 6 June 1944 — D-Day.
- 6 and 9 August 1945 — Hiroshima and Nagasaki.
The Second World War (1939–1945)
After Germany's invasion of Poland (1 September 1939), FDR proclaimed neutrality but engineered support for the Allies through the Cash and Carry Act (1939), Destroyers-for-Bases (September 1940), and the Lend-Lease Act (11 March 1941) — extending eventually about $50 billion in aid. The Atlantic Charter issued with Churchill on 14 August 1941 committed both nations to post-war principles.
Pearl Harbor and the Pacific War
The Japanese surprise attack on Pearl Harbor, Hawaii, on 7 December 1941 sank or damaged 19 American warships, destroyed 188 aircraft, and killed 2,403 Americans. FDR's "day which will live in infamy" speech secured a unanimous (save one) declaration of war. Germany and Italy declared war on the United States on 11 December.
Major Pacific battles:
| Battle | Date | Significance |
|---|---|---|
| Coral Sea | 4–8 May 1942 | First Japanese strategic check |
| Midway | 4–7 June 1942 | Decisive turning point; 4 Japanese carriers sunk |
| Guadalcanal | Aug 1942 – Feb 1943 | First major Allied land offensive |
| Tarawa | Nov 1943 | Bloody island-hopping |
| Saipan | June 1944 | Brings B-29 range to Japan |
| Leyte Gulf | 23–26 Oct 1944 | Largest naval battle ever |
| Iwo Jima | Feb–Mar 1945 | "Uncommon valor was a common virtue" |
| Okinawa | Apr–June 1945 | 14,000+ US dead; final stepping-stone |
The European theatre
US ground combat in Europe began with Operation Torch (8 Nov 1942) in North Africa, then Sicily (Jul 1943) and Italy (Sep 1943). The Normandy landings (D-Day, 6 June 1944) under General Dwight D. Eisenhower put 156,000 troops ashore. The Battle of the Bulge (16 Dec 1944 – 25 Jan 1945) was the last major German offensive. American forces met the Red Army at the Elbe River on 25 April 1945; Germany surrendered on 8 May 1945.
End of the war
FDR died on 12 April 1945, succeeded by Vice-President Harry S. Truman. After Japan ignored the Potsdam Declaration (26 July 1945), the US dropped atomic bombs on Hiroshima (6 August) and Nagasaki (9 August 1945), killing about 200,000 people. Japan surrendered on 15 August; the formal surrender was signed aboard USS Missouri on 2 September 1945.
For FPSC, three pairs anchor the era: 1917/1941 (US entry into each war), 1919/1945 (peace settlements), 1929/1933 (Crash / New Deal). For 1945, memorise the cluster: 8 May VE Day; 26 July Potsdam Declaration; 6 & 9 August atomic bombs; 15 August VJ Day; 2 September formal surrender; 24 October UN founded.
A new global role
By 1945 the United States produced about 50% of world GDP and possessed two-thirds of world gold reserves. The Bretton Woods Agreement (July 1944) had created the IMF and the World Bank. The United Nations Charter was signed at San Francisco on 26 June 1945. The Truman Doctrine (12 March 1947) and the Marshall Plan (1948) committed America to containment and to European reconstruction. Pearl Harbor had ended isolationism for good; for the rest of the twentieth century, the United States would be the central power in the international system.