This section of the Timeline of United States history concerns events from 1900 to 1929.

1900s

• 1900 - US population exceeds 75 million
• 1900 - Foraker Act
• 1900 - Gold Standard Act
• 1900 - US helps put down Boxer Rebellion in China
• 1901 - William McKinley assassinated
• 1901 - Theodore Roosevelt becomes President
• 1901 - US Steel founded by John Pierpont Morgan
• 1901 - Platt Amendment
• 1901 - Hay-Pauncefote Treaty
• 1902 - Drago Doctrine
• 1902 - First Rose Bowl game played
• 1902 - Newlands Reclamation Act
• 1903 - Great Train Robbery movie opens
• 1903 - Ford Motor Company formed
• 1903 - First World Series
• 1903 - Elkins Act
• 1903 - Big Stick Diplomacy
• 1903 - Hay-Bunau Varilla Treaty
• 1903 - Hay-Herran Treaty
• 1903 - Department of Commerce and Labor created
• 1904 - Roosevelt Corollary to Monroe Doctrine
• 1904 - Panama Canal Zone acquired
• 1905 - Niagara Falls conference
• 1905 - Industrial Workers of the World (IWW) formed
• 1906 - Algeciras Conference
• 1906 - Pure Food and Drug Act and Meat Inspection Act
• 1906 - Hepburn Act
• 1906 - Theodore Roosevelt negotiates Treaty of Portsmouth, receives Nobel Peace Prize

Teddy Roosevelt, the Bull Moose, led American progressives in the early 20th century

• 1907 - Gentleman's Agreement
• 1907 - Oklahoma becomes a state
• 1907 - Panic of 1907
• 1908 - Ford Model T appears on market
• 1908 - Root-Takahira agreement
• 1908 - Federal Bureau of Investigation established
• 1908 - Aldrich Vreeland Act
• 1909 - The U.S. penny is changed to the Abraham Lincoln design
• 1909 - William Howard Taft becomes President
• 1909 - Robert Peary plants American flag at North Pole
• 1909 - NAACP founded by W. E. B. DuBois
• 1909 - Payne-Aldrich tariff
• 1909 - Taft implements Dollar Diplomacy
• 1909 - Ballinger-Pinchot controversy

1910s

• 1910 - Boy Scouts of America chartered
• 1910 - Mann-Elkins Act
• 1910 - Mann Act
• 1911 - Supreme Court breaks up Standard Oil
• 1912 - RMS Titanic sinks
• 1912 - New Mexico and Arizona become states
• 1913 - Woodrow Wilson becomes President
• 1913 - Federal Reserve Act
• 1913 - 16th Amendment
• 1913 - 17th Amendment
• 1913 - Underwood tariff
• 1914 - Mother's Day created
• 1914 - Federal Trade Commission created
• 1914 - Clayton Antitrust Act
• 1914 - ABC Powers
• 1915 - The Birth of a Nation opens
• 1915 - RMS Lusitania sunk
• 1916 - US acquires Virgin Islands
• 1916 - Jeannette Rankin elected
• 1916 - Louis Brandeis appointed to Supreme Court
• 1916 - Adamson Railway Labor Act
• 1916 - Federal Farm Loan Act
• 1917 - Zimmerman telegram
• 1917 - US enters World War I
• 1917 - Espionage and Sedition Acts
• 1917 - Lansing-Ishii Agreement
• 1917 - U.S. Virgin Islands purchased from Denmark
• 1918 - President Wilson's Fourteen Points
• 1918 - Treaty of Versailles ends World War I
• 1919 - Senate rejects Treaty of Versailles and League of Nations
• 1919 - 18th Amendment
• 1919-1920 Red Scare (NOTE: the time that this event occurred can be debated. These are the two major years.)

1920s

Flappers symbolized the style and spirit of the Roaring '20s

• 1920 - 19th Amendment
• 1920 - U.S. population tops 100 million
• 1920 - Sacco and Vanzetti arrested
• 1920 - First radio broadcast in Pittsburgh, Pennsylvania
• 1920 - Volstead Act
• 1920 - Esch Cummins Act
• 1921 - Warren G. Harding becomes President
• 1921 - Washington Disarmament Conference
• 1921 - Emergency Quota Act
• 1922 - Fordney McCumber tariff
• 1923 - Warren G. Harding dies
• 1923 - Calvin Coolidge becomes President
• 1923 - Yankee Stadium opens
• 1923 - Teapot Dome Scandal
• 1924 - Immigration Act Basic Law
• 1924 - Indian Reorganization Act
• 1925 - Scopes trial
• 1925 - Nellie Tayloe Ross elected
• 1927 - Sacco and Vanzetti executed
• 1927 - Charles Lindbergh makes first trans-Atlantic flight
• 1927 - The Jazz Singer, the first "talkie" (motion picture with sound) is released
• 1927 - U.S. citizenship granted to inhabitants of U.S. Virgin Islands
• 1928 - Disney's Steamboat Willie opens
• 1928 - Kellogg-Briand Pact
• 1929 - Herbert Hoover becomes President
• 1929 - Saint Valentine's Day Massacre
• 1929 - Immigration Act
• 1929 - Great Depression begins
• 1929 - American Samoa officially becomes a U.S. territory