Democratic Peace Theory: Definition & Overview

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

There are many theories to explain why nations go to war, but how about one which explains why they don't? In this lesson, we'll check out the democratic peace theory and see how it has been applied in history.

The Democratic Peace Theory

In 1823, the United States announced one of its most important pieces of international policy: the Monroe Doctrine. In this policy, the US claimed that European aggression in the Americas would not be tolerated, because American democracies were fundamentally unique in the world.

There it is, all the way back in 1823. Americans have long promoted the idea that there is something special about a democracy, and that democratic governments and nations behave distinctly differently than non-democratic ones. As it turns out, they may have been on to something. In modern political science, a frequently debated concept is that of the democratic peace theory, which basically states that democratic nations are unlikely to go to war against each other. Unlike other theories which argue why nations become aggressive, this one seeks to explain why they don't, and the conclusion is the same that Monroe had back in the 19th century: democracies are unique.

Origins of the Democratic Peace Theory

So, where did this idea come from? Democratic peace theory as a formal idea was established in the 1970s, while the US was leading a coalition of mostly democratic nations against the authoritarian USSR. However, the basis for this theory is much, much older.

The first real articulation of the concept that democracies will not declare war on each other can be found in the writings of 18th-century German philosopher Immanuel Kant. Kant was a big supporter of the concept of representative government, and was watching as the American Revolution unfolded. In 1795 he published an essay entitled ''Perpetual Peace'', in which he claimed that a nation with a constitutional republic government would be very cautious about going to war.

Immanuel Kant

Why? Because declaring war requires the consent of the people, who would be the actual ones fighting in the conflict. Kant states that monarchs can declare war with little regard for personal safety because they won't be fighting, but a government of the people must take that decision more seriously. In Kant's opinion, therefore, a world full of constitutional republics would never know warfare. It would be a world of perpetual peace.


Kant's writing about democracy and warfare echoed across the years, before formally being incorporated as the democratic peace theory, which states that democracies are less likely to declare war on each other. Since the 1970s, researchers have pored over this theory to determine its credibility. The biggest piece of evidence they've used to support this theory: there haven't been any wars between democracies in the 20th century.

Let's take a look at the 1900s. The US started the century having just completed a war with Spain (a largely authoritarian empire) over the fate of its Cuban colony. Soon after, the various authoritarian empires of the world pulled each other into World War I, which the United States eventually joined as an associated semi-ally of the most democratic of these empires. After that, the world was divided into nations that were becoming more representative and democratic, and those that were becoming more authoritarian and fascist. That brought about World War II. Then, the US entered into the Cold War against the authoritarian USSR, and many democracies joined the American side.

For further evidence, many scholars have pointed out that there has been a general belief in democratic solidarity throughout the 20th century. At the end of World War I, Woodrow Wilson proposed the creation of the League of Nations. It was presumed that most democracies would join, and that encouraging the development of new democracies would prevent another world war. This exact same idea was applied after World War II to the creation of the United Nations. The UN is, in essence, an international institution built upon the concept that democracies don't want to go to war with each other. Furthermore, one of the UN's goals has always been to promote the growth of democracy around the world, as a way to prevent future conflict. There's still a belief that if every nation is a democracy, we can achieve global peace.

Signing the Declaration of the United Nations, which set the premise for what would later become the UN

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