- alliance: Agreement among nations to defend one another
- Blue Laws: Laws introduced by Progressives in the early 1900s designed to solve social problems, such as alcohol abuse
- Buffalo Soldiers: Nickname for African-American soldiers who fought in the wars against Native Americans living on the Great Plains during the 1870s
- conservation: protection and careful use of natural resources
- isolationism: policy in which a nation prefers to remain neutural and let other countries handle their own affairs
- isthmus: a narrow strip of land that connects two larger areas
- League of Nations: International organization formed after World War 1 to prevent wars
- muckracker: writer or journalist of the early 1900s who uncovered shameful conditions in business and other areas of American life.
- nationalism: Love of country and the desire to have one's country free from the control of another.
- Progressives: Reformers who worked to stop unfair practices by businesses and improve the way government works
- Rough Riders: volunteer soldiers led by Theodore Roosevelt during the Spanish-American War
- Spanish-American War: War between the United States and Spain in 1898 in which the United States gained Spanish territory
- Treaty of Versailles: Treaty signed in 1919 that officially ended World War 1.
- trust: Large companies with the power to drive out competition in an industry
- World War 1: War fought from 1914 to 1918 between the Central and Allied Powers. The United States joined the Allied Powers in 1917, helping them to victory.
- yellow journalism: false or exaggerated reporting