1: Unconditional support for Austria.The Kaiser promised Austria he would back them up no matter what, even though they made demands on Serbia that even the Kaiser thought were unreasonable.Austria would not have made such demands without the Kaiser's promise of unconditional support.
2: The German war plan.The Russians had a very large army, and an alliance with France, so the Germans felt they weren't strong enough to fight both at the same time.However, as Russia was so large and its transport infrastructure was underdeveloped compared to the other European powers, it would take 2 months for the Russian army to fully mobilize.The Germans therefore devised the Schlieffen Plan, which was a surprise attack on France through neutral Belgium.This plan was designed to outflank the French army and capture Paris