In more detail, after the archduke was assassinated, the Austria-Hungary Empire gave Serbia an ultimatum. Serbia refused, and so therefore the Empire declared war on them. The reason why so many countries were involved was because of alliances. Since Serbia is located in the Balkans, it is protected by Russia since they used to control that area. With Russia being on Serbia’s side, Russia declared war on the Empire. The Empire were friends with Germany so Germany helped the Empire. Russia was friends with France so they joined in. When Germany invaded France, Britain joined. The US would join years later.