Foreign Policy Research Institute A Nation Must Think Before it Acts Why did America Go to War in the 20th Century?

Why did America Go to War in the 20th Century?

Cite specific textual evidence to support analysis of primary and secondary sources.
Determine the central ideas or information of a primary or secondary source; provide an accurate summary of the source distinct from prior knowledge or opinions.
Integrate visual information (e.g., in charts, graphs, photographs, videos, or maps) with other information in print and digital texts.
Distinguish among fact, opinion, and reasoned judgment in a text.
Analyze the relationship between a primary and secondary source on the same topic.

Students will be able to evaluate the reasons why America became involved in World War I, World War II, the Korean War, the Vietnam War, and the Persian Gulf War, by applying the theories of the different thinkers to the different 20th century conflicts and using these theories to explain America’s rationale for becoming involved.

1. This should be part of a larger unit on the study of modern warfare.
2. Students will begin by studying the theories of the different thinkers as to why nations go to war.
3. Students will study the documents and, for each, use specific textual evidence to explain why this was a cause for US involvement in the conflict.
4. Teacher will model the first document out loud and demonstrate how to use information from the text to explain the reason for US entry into the conflict.
5. In pairs or small groups, students will read and analyze the next document, with the teacher’s guidance, and then explain, in their own words, how the text explains proves America’s reasons for entering the war.
6. Students will then discuss in groups which historian and theory this reason for war appears to match.
7. Students will then use a variety of resources including the documents provided, as well as those found inside of their classroom, library, and the Internet, to research the reasons for US involvement in the different 20th Century conflicts.
8. Students will use this information to write an argumentative essay in which they must prove that America’s reasons and rationale for entering all the major 20th Century conflicts matches the theory of the thinker that they chose.

Teacher will model the use of the Document Analysis worksheets, use highlighters for the analysis and demonstrate how to complete other activities in either pairs or groups.

Students will apply the theories of warfare to the current conflicts in which the United States is involved, e.g., the wars in Iraq, Syria, and Afghanistan.

Students will hold a panel discussion/debate on the theories of why America went to war and the evidence that proves the validity of each argument.

FPRI References
Thinking About War

Thinking About War

World War II

World War II (1941-1945)

The Korean War

The Korean War (1950-1953)

The Vietnam War

The Vietnam War (1964-1973)

The Persian Gulf War

The Persian Gulf War (1990-1991)

US Entry into World War I

America and World War I

America and World War One

The US in World War I (

World War II History (

US Entry into World War II

The National World War II Museum

Facts & Summary

The War Begins

TIME Magazine Article on Korean War

Fast Facts

Vietnam War History

The Road to War

Best Vietnam Websites

Persian Gulf War History – Facts & Summary

PBS – Frontline
National Archives

  • David Lonborg
  • George J. Ryan Middle School 216
Related History Institute
Grade Level
  • Middle School: 8
Time Frame
  • 2-3 Weeks

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