After the assassination of the archduke, Austria-Hungary decided that they needed to show more authority to maintain their credibility as a strong force in the Balkan region. They eventually required help from Germany to protect them from the threat of Russian intervention. 

 

On July 23, 1914, the ambassador of Austria-Hungary demanded that the Serbian governemnt took steps to eliminate terrorist gatherings within the borders, allow for an independent investigation on the assassination, and no longer creat anti-Austria-Hungary propoganda. If Serbia did not follow, the result would lead to military action. Serbia then looked to Russia for help.

Allied with Germany, Austria-Hungary declared war on Serbia on July 28, 1914. After hearing news of Russia's mobilization, Germany also declared war on Russia on August 1. They launced an attack on France, who was allied with Russia. During this attack, the Germans went through Belgium, violating their neutrality. Belgium was allied with Great Britain, which brought them to the war a well.