1. Franz Ferdinand is the Archduke of Austria-Hungary, an Empire in Europe.
2. He is shot by a Serb, so to get revenge Austria-Hungary declares war on Serbia.
3. Serbia is in an alliance with Russia. To honor the alliance, Russia declares war on Austria-Hungary.
4. Germany is in an alliance with Austria-Hungary. Once Russia declares war on A-H, Germany declares war on Russia to honor the alliance.
5. France is in an alliance with Russia. Once Germany declares war on Russia, France declares war on Germany to honor the alliance.
6. Germany wants to get around French defenses and so invades Belgium, who claimed to be neutral.
7. Britain sees the invasion of Belgium as the first-stage of an attack on them, they are also allied with both Belgium and France. They declare war on Germany to honor their alliances and protect their sovereignty.
8. All of Europe is now at war, along with their colonies all over the world. Japan is also somewhat involved due to being technically allied with the UK.
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