The Serbians never expected an attack from the Germans. They assumed that the other three armies near the Odili (Austria) capital, Vienna, were stronger and posed a greater threat—especially the Zeke (Czechoslovak) army stationed on the border. They thought the Germans would naturally strike them first.
Yet the Germans, led by Mainz, took the unexpected approach: instead of engaging the nearest enemy, they moved to attack the Serbians.
Thanks to the cooperation and intelligence from the local population, the German forces blocked all escape routes and began bombarding the Serbian camp. The Serbian soldiers, untrained and unprepared for years, quickly descended into chaos. Some tried to resist, while others attempted to flee with whatever belongings they could carry.
The battle was over in just half an hour. More than 15,000 Serbian troops were annihilated, with none captured.
"Your Excellency, you are truly clever," Guderian said. "Had we not prepared the encirclement in advance, they might have escaped."
Indeed, the Serbians, whose homeland lay to the south, would have fled in that direction under normal circumstances. But fearing reprisals at home, they chose to move northward to join the other three armies, seeking safety in numbers. Mainz had anticipated this, ensuring their escape routes were cut off.
Guderian, though impressed, admitted, "The lion fighting a rabbit still requires my full attention. I was too optimistic before!"
Mainz's strategic foresight ensured the battle's success, and Guderian recognized the importance of humility and careful planning in war.
After defeating the Serbians, Mainz pressed on. On 3 October, he routed the 50,000-strong Zeke army along the southern bank of the Danube. Unlike the Serbians, the Zeke were not traitors—they were a regular army, though they had no real desire to fight for the Entente. Mainz's strategy was to deliver a decisive blow, forcing them back with heavy losses rather than complete annihilation.
Under cover of night, the Germans advanced quietly, followed by rapid artillery fire and a sudden assault. The Zeke commander, misjudging the German strength at 150,000 troops, panicked and ordered a full retreat. His forces fled 120 miles to the Zeke homeland, losing over 15,000 men and roughly 80% of their weapons in a single night.
The Zeke had no appetite to counterattack; they reinforced their borders but would not advance into Austria again. With the strongest army now withdrawn, only the Croatian and Slovenian forces remained. The situation for Austria was becoming increasingly clear.