Final answer:
Austria-Hungary declared war on Serbia on July 28, 1914, after Serbia could not agree to all of their ultimatums, setting off a series of events that led to World War I.
Step-by-step explanation:
When Serbia was unable to fully comply with the ultimatums during the July Crisis of 1914, Austria-Hungary declared war on Serbia. This marked the onset of hostilities that would escalate into World War I. The response from Austria-Hungary came after Serbia agreed to most but not all demands; the refusal to allow Austrian investigators to operate unrestricted within Serbian borders being a sticking point for the Serbian government. Austria-Hungary felt it was essential to punish Serbia for their perceived role in the assassination of Archduke Franz Ferdinand. The demands included suppression of anti-Austrian propaganda and activities, the dissolution of Serbian nationalist organizations, and the arrest of those involved in the assassination plot against the Archduke.
Despite the looming threat and Germany's promise of unconditional support to Austria-Hungary—often referred to as the "blank check"—Serbia could not acquiesce to what it viewed as a violation of its sovereignty. Consequently, Austria-Hungary initiated military action, setting off a chain reaction among the European powers. As a result, Russia, who had a treaty with Serbia, began to mobilize, followed by Austria-Hungary's ally, Germany, which declared war on Russia in support of Austria-Hungary. The complex web of alliances pulled in more nations, with France backing Russia, and eventually, Germany's violation of Belgian neutrality brought Britain into the war.