Conflicts over alliances dragged all powers in, creating a global conflict
A great majority of world leaders at the time were related, and more notably, their nations were all intertwined through a system of tight treaties and agreements in which actors would fearlessly support those they had agreed to flank in conflict.
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The majority of European powers began forming tight-knit alliances, forming into distinct sides. Essentially, it was the Triple Alliance including Germany, Austria-Hungary, and Italy against the Triple Entente, including Britain, France, and Russia. Additionally, powers in these alliances also had agreements with smaller actors like Serbia and Belgium. Germany and Austria-Hungary inciting conflicts in these less influential territories threw the rest of the alliances into the mix. Britain had sworn to help maintain Belgium's independence, so they rushed towards their defense. Russia, an ally to Serbia, was furious when Austria Hungary took over Bosnia (a province containing many Serbs), adding flame to the fire. The interconnectedness of the European theater, like a flailing octopus with many tentacles, resulted in the confusing mess that became World War 1.
Although many nations had allyships, the German and Austrian Dual Alliance is most responsible for the war because Germany had many enemies and only a single Ally. The Dual Alliance caused the initial conflict/incitement of violence with Serbia and Russia, which resulted in other powers getting involved.