What if Russia did not declare war on Austria-Hungary in 1914?
Had Russia decided to stay out of the war then the guns of August would have fired on a much reduced theater. There would be no Great War. Without Russia coming to the aid of Serbia as the great protector of all Slavic peoples, Germany would not have entered the war. No Germany, no France, no intrusion on Belgian neutrality, no Britain, no Great War. One event set off the other in a chain reaction that turned a simple regional tussle into a global disaster that destroyed the old world order and provided the impetus for the birth of communism and Nazism.
So why did Russia go to war? Because they saw themselves as big brother to tiny little Serbia. Serbia is a newcomer on the world stage, and they might seem weak after fighting two wars in the previous two years. Austria was a great power, allegedly, and they were itching for a war with Serbia for years. The death of Franz Ferdinand was just a pretext. The ultimatum that was delivered to Serbia to avoid war was designed to be unacceptable. Austrian brass knew Serbia would never capitulate to so many outrageous demands. Everyone else knew so too, and even the Kaiser begged Franz Joseph to be more reasonable.
What everyone knew turned out to be wrong. Serbia, eager to avoid war, did capitulate to the shopping list of unreasonable demands. Unfortunately they were about 20 minutes late in delivering the telegram, so Austria went to war. The Kaiser pleaded again, but the Austrians would hear none of it.
Now this clearly appears to be a David versus Goliath situation, and I would agree, but one needs to remember that David won with a single shot! The story of David and Goliath is that of a foe who appears to be invincible but is really fatally flawed, versus an opponent who appears to be small and weak but who is really swift and brilliant, and capable of delivering a fatal blow before the enemy has time to react. While on paper Serbia looked hopelessly outclassed, the reality was the exact opposite.
The Austrian military was weak, calcified, and decrepit. There was no cohesion within the empire, that was threatening to rend itself in three directions. Austria-Hungary really was a very sick empire. The Hungarians constantly tried to stonewall every piece of legislation the Austrians tried to pass. Emperor Franz Joseph, then age 84, had sat on the throne for 66 years! The people considered him to be the immortal Emperor, because multiple generations had lived and died under his rule. There was hardly a subject alive who remembered a time before this tired old man ruled. Several times he dissolved the Hungarian government to curb their defiance, and it was only he who could hold the empire together. The moment the emperor took his last breath it would only be months before three nationalities who were constantly at each other's throats would tear the empire apart.
Franz Joseph lived to see Austria's fall from grace in the Austro-Prussian War. Prussia was now the new head of the German peoples, and their military seemed unstoppable. Austria was old, and empire of geriatrics, unable to think outside of the mouldy box that had been stored in an antiquated cedar chest for centuries. Reform was impossible, and, as a result, Austria would never be able to adapt to modern warfare.
Serbia, on the other hand, was just the opposite. The Kingdom of Serbia was young and new, and, unlike most other European nation states, was ruled by a Serb king, not a German! Instead of approaching the Germans on bent knee, begging for an inbred king, the Serbs pulled one of their own out of the mud, a peasant farmer, and made him king. A full-blood Serb ruled over the Serbs, and the people were beaming with pride.
Time and again we see that superior numbers and equipment is no match for superior training and will to fight. Serbia looked weak after fighting to Balkan Wars. Their territory had increased dramatically, and it appeared that they could not sustain a fight against a far superior enemy. The treasury had been drained, and supplies were short. Austria attacked with twice the number of men, with far better equipment and supplies, and a whole war industry to support it. It should have been an easy victory for Austria, which is why Russia decided to intervene on Serbia's behalf.
That intervention might not have been necessary, and, either way, ultimately led to the near total destruction of Serbia by involving it in a larger war.
The Austrian brass, commanded by General Oskar Potiorek, suffered from impulsivity, lack of imagination, and near incompetence. Potiorek made a disastrous initial move that was so poorly conceived that the Serbians initially didn't believe the Austrian army would commit itself to an attack in the mountains where maneuver was impossible. After four days of fierce fighting Serbia emerged victorious, pushing the Austrians back.
Austria would hammer again, hoping to use their numbers to wear the Serbians down. With little in the way of ammunition, Serbia fell back in a strategic retreat, leading the Austrians into a trap. The Serbian army suffered, to be sure, from lack of supplies, fatigue, and poor weather, but morale held, and the scorched earth policy hampered the Austrian advance. By the end of November the Austrian army found itself in barren countryside, surrounded by mountains and fortifications Serbia had prepared months in advance. Finally, under cover of a heavy storm, the two armies clashed on the banks of the Kolubara River. Like the Spartans at Thermopylae, the Serbian army held its ground in the south, while strategically withdrawing from Belgrade in the north.
Thinking they were winning, and fearing the army would outstrip its supply line, the Austrians halted. Seizing the initiative, the Serbians moved in to encircle the Austrian army, which had failed to prepare for a counter-attack. Serbian morale soared. Serbia advanced behind the Austrian lines, capturing tens of thousands of Austrians in a ten day offensive, suffering few casualties. By 15 December the Austrian army had retreated fully across the Danube.
Austria-Hungary had been soundly humiliated. They had failed in all their objectives. This tiny upstart had routed their massive and majestic army that was once the pride of Europe for centuries. Appalled by the atrocities committed by the Austrians, and inspired by the tremendous victory of so few fighting against so many, nations all over the world rallied to Serbia's cause.
Serbia had suffered tremendous losses, to their army and civilians, but they had emerged victorious against one of the great powers. They were jubilant, proud, and admired around the world. Had this been a regional war Serbia could have rested on its laurels as a rising power who had just won a seemingly impossible victory. Austria-Hungary would be left sulking, and may have to deal with insurrection in Bosnia if the people were rallied by the great Slavic victory. Had this been a regional war it would have ended here, status quo ante bellum.
But this was not a regional war, this was a world war, and it was far from over.
Nearly one year later, in October 1915, Austria-Hungary, backed by Germany and Bulgaria, launched a two-pronged invasion of little Serbia. Unable to fend off attacks from three armies, the Serbian army retreated through a very hostile Albania, losing hundreds of thousands of men to privation, terrible weather, disease epidemic, and constant harassment. Hundreds of thousands more civilians would die under an Austrian policy of genocide. By war's end Serbia had lost one quarter of its population.