Theodore Roosevelt riding on horseback with Kaiser Wilhelm II

When the Russians came to the defense of the Serbs, the Germans had no choice but to go to war to defend their Austrian allies. Their strategy for war against Russia was the Schefffein Plan which called for an initial attack against France, Russia's ally. So even though Germany had no dispute with France at the time, it still declared war against France.

The declaration of war by Austria on Serbia did not guarantee that a more massive war had to occur.
On July 29th, Winston Churchill suggested to the British cabinet that a conference of the sovereigns of all the countries involved be held to resolve the differences. However, the German felt they would have little choice to intervene if Russia honored its treaty with Serbia. The Germans knew that if they went to war with Russia, their plans called first to attack France. What they did not want to do was also end up in a war with England. The Kaiser’s brother called on his cousin King George V, who reportedly told him that England would try to remain neutral in any coming war. Thus the Germans felt confident that if war came and they attacked France, England would not come to its aid.

The leaders tried to stop the march to war, but by now, their populations were enthusiastic about the thought of war. Nothing in their past had prepared the people of Germany, France, or Russia how devastating an all-out war could be. When on July 29th Austrian ships had opened fire on Belgrade, the Russian Tsar telegraphed the German Kaiser and wrote: “To try to avoid such a calamity as a European War, I beg you in the name of our friendship to what you can to stop your allies from going to far.” The Kaiser responded that he was doing all he could to restrain them.
The Tzar went as far as to order his army to stop their mobilization. The Kaiser tried to respond in kind, but his army refused to stop their mobilization, and the Russian, in turn, continued theirs. As Germany prepared to go to war with Russia, it tried to avoid conflict with France. It asked the French if they were willing to forgo their Alliance with Russia and stay out of the war. When the French responded that their treaty of Alliance bound them, the German concluded they had no choice, and thus they prepared to go to war with France.
On August 1, Germany declared war on Russia. When France began mobilizing, the German moved troops into critical positions in Luxembourg to provide a starting point for an invasion of France. On August 3rd, the Germans declared war on France.