On July 29, 1914, Austro-Hungarian river monitors opened fire on Belgrade, Serbia, one month after Archduke Franz Ferdinand's assassination in Sarajevo. Austria-Hungary had declared war on Serbia the previous day following Serbia's rejection of key ultimatum terms. The bombardment of Belgrade, situated directly across the Danube from Austro-Hungarian territory, triggered the cascade of alliance obligations that drew Europe's powers into conflict within days. Russia mobilized to defend Serbia, prompting Germany to declare war on Russia on August 1 and on France on August 3.