The combat of Laa (9 July 1809) was one of a number of minor clashes between the French and the retreating Austrians in the aftermath of the battle of Wagram, and helped the French identify the main Austrian line of retreat. In the aftermath of that battle most of the Austrian army retreated up the highway that led to Znaim, but Rosenberg's IV Corps was forced to move on a line further to the east. On the night after the battle Rosenberg reached Bockfliess, then moved onto the main road to Brünn. After advancing part of the way up that road he branched off to the left and advanced through Mistelbach and Staatz up to Laa on the Thaya River, from where he could guard the left flank of the main Austrian army on its way to Znaim. This move angered Archduke Charles, and he ordered Rosenberg move back to the east, to protect the road to Brünn.
While the Austrians were retreating north Napoleon was busy attempting to discover which road they had taken. One of the forces sent to achieve this, under General Marmont, was sent up the same highway to Brünn, before also turning off towards Laa, leaving Davout to follow the highway. On 9 July Marmont's cavalry, which consisted of Montbrun's light cavalry division from Davout's corps and the Bavaria cavalry, ran into Radetzky's cavalry from Rosenberg's corps. Radetzky was able to shield the rest of the corps as it marched away to the north-east, down the Thaya River towards Davout's advancing columns before escaping himself.
Marmont crossed to the north bank of the Thaya, just as a fresh Austrian force, under General von Alstern (from Hohenzollern's II Corps) arrived from the west. Alstern realised he was badly outnumbered and immediately retreated north-west up the river towards Znaim. Marmont incorrectly identified Alstern's men as being from Rosenberg's corps, and reported to Napoleon that the entire IV corps was retreating towards Znaim, thus correctly identifying the main Austrian line of retreat, although for the wrong reasons. Marmont then continued up the river towards Znaim himself, where on the following day he began the battle of Znaim (10-11 July 1809) by attacking what he believed to be the Austrian rearguard..