When the Prussian army began its invasion of Saxony, it had two major goals envisioned: The first, to destroy the Saxon army before it can be relieved by Austria; the second, to advance towards Bohemia and capture the two major fortresses of Prague and Königgrätz before the onset of winter.
In his misguided aspiration to achieve these objectives, Frederick's plans for September called for the main body of the Elbe Armee in Dresden to advance past the Saxon encampment in Pirna, seizing the road leading from Dippoldiswalde and advancing towards the banks of the Eger river near Lobositz and Brux. By doing so, the Elbe Armee hoped to isolate the Pirna Camp and gain strong defensive positions against the inevitable Austrian counterattack by forcing a river crossing.
Wedell's column, located across the Elbe river in Bautzen, would secure the crossroads and bridge east of Pirna, then turn southward to advance along the Elbe's eastern bank, allowing it to scout enemy positions near Prague and possibly endanger the flank of the Austrian army. Meanwhile, Schwerin's Schlesien Armee will advance directly from its position in Glatz to Königgrätz, laying siege to the fortress. Once Königgrätz is taken, Schwerin could proceed towards Prague.
The first two weeks of September go mostly according to plan. The Elbe Armee leaves behind Prince Wilhelm to directly besiege Pirna and Moritz to guard Dippoldiswalde, then advances towards Lobositz, where it briefly meets the Advance Guard of the Austrian army. The latter force, unwilling to give battle against the bulk of the Prussian army on its own, retreats behind the Eger without engaging. Schwerin advances quickly, encountering no opposition and laying siege to Königgrätz. The cavalry detachments of Karl von Katte and Friedrich Leopold Gessler link up near Bautzen to form a cavalry corps of some 12,000 men in support of Schwerin's army.
The only engagement during the first half of September occurs as the culmination of a raid by Hans von Zieten's two Hussars battalions against an Austrian supply depot in Karlsbad. After crossing the mountains between Chemnitz and Karlsbad, the Hussars assault the depot, hoping to capture it in a coup de main. However, the garrisoning infantry company takes up defensive positions inside the depot and, despite suffering heavy casualties, thwart Zieten's attempts to capture it.
As the third week of September starts, Wedell has successfully secured the area east of Pirna and begins moving southwards. Meanwhile, Ferdinand von Brunswick's column is ordered to advance towards Brux, where it can block the Austrian army from attempting to outflank the Elbe Armee in Lobositz. This would become the
second mistake of the Kingdom of Prussia.
Towards the end of the month, leading units of the Austrian army commanded by Maximilian Browne, moving faster than anticipated by the Prussian General Staff, crossed the river into Brux in the course of their well-predicted flanking attempt. It is not clear whether Ferdinand disregarded, misinterpreted or never received Frederick's explicit orders to assume defensive positions, but as his column enters Brux on September 25th, Ferdinand immediately moves to attack the Austrian army, beginning the first major battle of the war.
Taking command of the battle, Frederick barely managed to hold the Prussian forces together long enough for the columns of Keith and Moritz, force-marching to join the battle, to arrive and even the odds. However, the damage incurred to Ferdinand's column by this point is too great, and shortly after the arrival of reinforcements it withdraws from the field. Once again outnumbered by the Austrians, the Elbe Armee makes a valiant stand before effecting a general retreat towards Lobositz.
Though total forces in the Battle of Brux prove to be nearly equal, with both armies fielding approximately 45,000 men, poor coordination on the Prussian side allowed the Austrians to enjoy at least a two-to-one advantage in numbers for the majority of the engagement. This, in addition to Ferdinand's decision to attack the Austrians rather than take up defensive positions, have culminated in a clear strategic defeat. However, the well-trained Prussian units have managed to extract a heavy toll from the Austrians; casualties on both sides are high, with Austria losing nearly 12,000 men to the Prussians' 10,000.
With both sides exhausted by the bloody battle, September of 1756 ends as both the Prussian and Austrian armies stop to recover and reconsider their operational plans.