I've frequently heard that the ultimatum was designed to be impossible to accept, thus forcing a war. When looking up details though, it doesn't seem all that harsh in my opinion. From what I can tell, it required Serbia to allow Austria-Hungary to open an investigation into the assassination, forced Serbia to stop anti-Austrian propoganda, and required Serbia to go after anti-Austrian terrorist organizations. This seems like a fairly reasonable response, so I don't see why Serbia wouldn't just give in to the demands to avoid war?
The ultimatum called for Austrian police to have free reign over all of Serbia, which would essentially negate Serbian sovereignty as an independent state if foreign, hostile forces could go anywhere and do anything they wished with no interference from Serbian authorities.
This seems like a fairly reasonable response, so I don't see why Serbia wouldn't just give in to the demands to avoid war?
They did accept the Ultimatum, with qualifications. They would turn over the officers demanded in the Ultimatum, but requested that a neutral party be allowed to conduct the investigation, specifically the International Court of Justice at the Hague. The issue was that this was not the response that the Austro-Hungarians had banked on. Berchtold, the Foreign Minister, had made it clear that the demands were to be unacceptable, so as to prompt Serbia's rejection, and thus give the Austro-Hungarians an excuse to go to war. This turned out to not be the case, but the Austro-Hungarians shelled Belgrade anyways.
The bottom line is that it didn't matter what response the Serbs gave; the Austro-Hungarian leadership had it's hearts set on a war to secure their position in the Balkans, and especially considering the support they had from the Germans, there was little that could stop them.
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