Austria had an alliance with Germany prior to World War I which gave them assurance they would be protected if war was needed. Serbia had the backing of Russia and therefore Austria was going to need more than their own resources if they were to invade Serbia in retaliation for the assassination of Archduke Franz Ferdinand. Germany gave Austria a "blank check" meaning they would offer any and all assistance needed against Serbia and their allies. With German backing, Austria was more confident in issuing the ultimatum, which they knew would be rejected. Declaration of war triggered the complex alliance system in place in Europe prior to WWI.
