Why did World War 1 happen? This is the question addressed in Richard Maybury’s book on World War 1. Link: World War I. The rest of the story and how it affects you today

Firstly, the belief in deadly ideas that when widely adopted lead to war. These deadly ideas are:

Political power. Political power is the legal privilege of using force on others who have not harmed anyone.

The idea of the Pax Romana. The belief in the Pax Romana is that there was that there was peace in the Roman empire due to a strong central government.

The glory of war. Propaganda teaches that war is glorious. It makes the soldiers want to go to war.

The following ideas are more U.S. specific. They explain why the U.S. got involved in world war 1:

Global protection. Global protection is the belief that wherever their citizens go they will be protected by the government.

Interest. Interest is undefined in law and is used as a reason to go to war.

Manifest Destiny. Manifest destiny is the belief that god gave U.S. the permission to rule all land. Even if other people already live there.

World war 1 also, happened due to existing international tensions and alliances. Due to alliances small wars that otherwise would have stayed small became a world war. An alliance is the promise to help others in war. Horrible crises before the first world war were:

The Moroccan crisis. As a result of the Moroccan crisis the hatred between France and Germany had grown and the friendship between France and England had strengthened.

The Boxer Rebellion and the Russo-Japanese war. In the Boxer Rebellion the Chinese rebelled against European rule and the Japanese started a war for Manchuria and Korea against Russia. It was the start of Japan building an empire, just like the European powers.

In 1902 Japan and Britain made an alliance. This required Japan and Great Britain to support each other if one became involved in a war with more than one power.

To summarize: World War 1 happened, because of the adoption of deadly ideas that lead to all kind of crises and then escalated into a world war due to international alliances.