In an attempt to reunite with the French Army (currently to the west of Abensberg) and prevent the Austrians from trapping it against the Danube, the French 3rd Corps under Davout probes south towards the village of Thalmessing. The Austrians, led personally by the Archduke Charles, are probing north, trying to close the noose around Davout by pinning him against the Danube and isolating the 3rd Corps from the main French army. On the evening of April 18, 1809, the Austrian Advance Guard under Somariva skirmishes with the Bavarian division under Wrede near the village of Thalmessing. At nightfall, the Bavarians were left in control of the village as Davout's Corps reinforced Wrede on the north bank of the Thalmessing stream, while Charles concentrated both Rosenberg's 4th Corps and Liechtenstein's Reserve Corps on the south bank of the stream in anticipation of tomorrow's upcoming clash with Davout.

As the morning mist lifted, the two armies were arrayed as follows: The French right was held by the Bavarians under Wrede, the French center was held by Morand, the left held by Friant, and the two cavalry divisions held in reserve behind Thalmessing. The combined French and Bavarian forces in and around Thalmessing totaled 28,320 foot; 3,840 horse; and 6 batteries.

Charles deployed Liechtenstein's Reserve Corps on the Austrian left, Bartenstein and Dedovich's divisions in the center, and Somariva division on the right. The Austrians mustered 30,240 foot; 4,160 horse; and 8 batteries.

At 8:00 am on 19 April, Davout, having gained the initiative, issues orders to his staff and quickly sets the 3rd Corps into motion. Montbrun's light cavalry division, originally deployed on the French left begins to move towards the French center behind Thalmessing as the cannonades from French and Austrian guns erupt. The French gunners quickly damage Somariva's battery and one of the 12lb Corps batteries while forcing the other Austrian Corps 12lb battery and Dedovich's battery to limber and retreat towards safety. The Austrians for their efforts damage one of the French 12lb Corps batteries deployed in front of Thalmessing.

At 8:30, Dedovich accepts orders from Rosenberg to advance on the French center and engage the French deployed in front of Thalmessing. Dedovich quickly appraises the situation and sets his division into motion before either Bartenstein and Somariva receive orders from Rosenberg. As the cannonade continues, one Austrian 121b battery is destroyed, an Austrian battery damages Morand's battery in counter battery fire, and one of the French Corps batteries limbers and falls back to Thalmessing. Montbrun's cavalry joins the Bavarians on the French right, as St. Sulpice's heavy cavalry division moves west along the ridge line towards the French right.

At 9:30, the Austrians gain the initiative and Dedovich presses on towards the French center, nearly reaching the stream and bridges, while Bartenstein accepts his orders to engage and steps off on Dedovich's left. On the Austrian right, Somariva accepts his engage orders and directs the Grenz regiments to descend the heights and advance on Friant's positions. Friant's 8lb battery pours deadly fire into the advancing Deutsch-Banater Grenz causing 600 casualties. The Grenz hold their ground in disorder while the Vincent Chevaulegers charge the 33rd. The chevaulegers become disordered as they cross the stream, then suffer 300 casualties when the 33rd unleashes a well-timed volley. When the volley fails to stop the oncoming chevaulegers, the 33rd falls back before the chevaulegers can make contact. Disordered, the chevaulegers continue their assault riding over one of the French Corps 12lb batteries before recalling back to the south bank of the stream.

On the Austrian left, Hessen-Homburg's batteries concentrate their fire on the Bavarian battery. The Austrian batteries first suppress, and then destroy the Bavarian guns. The French briefly regain the initiative at 10:30 and take the opportunity to deploy the Bavarian cavalry and both French cavalry divisions on the north bank of the stream threatening Liechtenstein's Reserve Corps on the Austrian left. The Austrian grenadiers form square in the face of the substantial cavalry threat.

By 11:00, the Austrians regain the initiative and begin their assault on the French positions in earnest. The Ludwig IR#8 close assaults the 108th but is repulsed suffering 200 casualties.

After seeing the Ludwig IR's assault thwarted, Dedovich orders the Koburg IR#22 to assault the 108th. IR#22 goes in against the 108th and the 108th is forced to fall back. IR#22 suffers 100 casualties while the 108th loses 300 men in the assault. Meanwhile on the Austrian left, the Franz Cuirassiers suffer 300 casualties when it becomes the primary target of an artillery barrage as the French begin to shift their batteries to the west opposite the bridges. As Bartenstein's division nears the stream, Chasteler's I R#46 is ordered to close assault the 61st on the opposite bank of the stream. The 61 st falls back as the French left retrogrades towards the center and behind Thalmessing.

The French keep up a lively harassing fire as they withdraw inflicting casualties along the Austrian line. Czartoryski IR#9 suffers 600 casualties as it receives the brunt of the French musketry fusillade. The remaining French 121b battery in the center routs as the French infantry pull back. The French 61st loses an additional 200 casualties to Austrian artillery barrage as it fails to fall back out of effective artillery range. On the Austrian left, the Bavarians advance on the Austrian positions with Minuzzi's brigade suffering 600 casualties to artillery fire.

Unable to take the punishment from the Austrian batteries, by 1:00pm the Bavarians begin to withdraw back to the heights as the French center continues to pull back and shift towards the French right. The French 65th takes small arms fire and routs towards Thalmessing, carrying the 30th with it. The Austrians seize this opportunity to assault the French positions. As Somariva's advance on the Austrian right renews, the 12th Grenz assaults the 15th Legere. The 12th Grenz loses 400 casualties and retires. Frustrated by the failure of the 12th's assault, Somariva orders the 13th Grenz to assault the 15th Legere. The 13th Grenz likewise fails and retreats back across the stream. The 15th Legere suffers 400 casualties in the two Austrian assaults. As the French left weakens both from voluntary and involuntary means, IR#22 assaults the 48th line, suffering 300 casualties, yet inflicting 700 casualties on the 48th and forcing it to retreat. The Reuss-Greutz IR#55 assaults the 33rd, taking 500 casualties and dishing out 300. The 55th's resolve falters and it falls back in disorder, to be replaced in line by the IR#22. IR#22 approaches the 33rd and then hits it inflicting 500 casualties and forcing the 33rd to retreat back into the 48th.

The 48th, weakened by IR#22's earlier assault, breaks and retreats back beyond Thalmessing. Bellegarde's IR#44 falls back due to small arms fire, taking 200 casualties. Somariva crosses the stream in strength in pursuit of the withdrawing Morand. Elements of Bartenstein's division re-deploys to face the threat by French cavalry on the Austrian left.

Just as the Austrian assault was gaining momentum in the center, the French re-gain the initiative at 3:00pm. Davout launches the cavalry divisions into Liechtenstein's Reserve Corps on the Austrian left, in an attempt to open a line of communication back to the main French body. In the initial assault, the Clement cuirassiers clash with the Albert cuirassiers. The Clement cuirassiers suffer 80 casualties and send the Albert cuirassiers routing with 320 casualties. The Pajol hussars charge Bartenstein's battery killing 40 gunners before falling back. Pire's hussars charge the IR#46 as it forms square. The square of IR#46 holds.

Pire loses 80 troopers and IR#46 loses 200 men. Guiton's cuirassiers clash with the Reisch dragoons, causing 320 casualties while taking 240 casualties. The Reisch dragoons fall back in disorder and the Guiton cuirassiers recall to reform on the Austrian left flank. The Bavarian chevaulegers charge two Austrian fast foot batteries on the heights and are repulsed, suffering 240 casualties. The Austrian center and right under Somariva and Dedovich continue pressing the French back through Thalmessing.

Still holding the initiative, the French at 4:30pm again launch cavalry assaults on the Austrian left flank in an attempt to cover the withdrawal of Wrede and Friant's divisions across to the southern bank of the stream. The 5th cuirassiers overrun the Austrian fast foot battery on the ridge and then retire after suffering 80 casualties. The 8th hussars go in against d'Aspre's grenadiers catching them in line. The hussars suffer 160 casualties but put the grenadiers to flight causing 700 casualties.

At 5:30pm, the Austrians regain the initiative. In an attempt to stop the French from reinforcing their successes on the south bank, the Rohan grenadiers close assault Becker's Bavarian brigade. The Bavarians suffer 200 casualties and retreat carrying the 48th with them. Although the grenadiers open a hole in the French line, they become surrounded by the French as more French units pour across the stream. With no means for escape, the Rohan grenadiers lose 480 men in a fire fight with the 111th. To the northwest of Thalmessing, the Vincent chevaulegers charge the 30th. The 30th retreats suffering 100 casualties. Victorious, the Vincent chevaulegers then hits the 61st, forcing it and Morand's battery back. IR#40 assaults the 17th inflicting 360 casualties while losing 480 casualties. The 17th falls back. At this point, the game is halted.

Aftermath

As French units stream to the south, the Austrians press against the French batteries trapped on the north bank of the stream. Liechtenstein's Corps, weakened by repeated cavalry assaults cannot close the noose around the fleeing French. Charles, exhausted from the battle, is unable to pursue. The French lose 6,400 foot; 1,200 horse; and 6 batteries in the engagement. The Austrians lose 10,200 foot; 1,920 horse; and 3 batteries in the action. The Austrians capitalized on their ability to maintain the initiative throughout the morning by pressing the French center and left with good results. When the initiative swung back to the French during the afternoon, the Austrian hopes of crushing Davout evaporated.

The French decision to shift the cavalry to the right flank and then pressure the Austrian left while Friant and Morand's divisions conduct a fighting withdrawal proved decisive. With the Austrian command and staff not the equal of their French counterparts, the Austrian divisional commanders were forced to commit their regiments in piecemeal assaults on the retiring French. These piecemeal Austrian assaults not only allowed the French to withdraw before the Austrian's could gain local superiority in the center and Austrian right, but also kept French casualties low in relation to their opponents. Although the Austrian assaults of Dedovich and Somariva were successful, the Austrians paid dearly for the ground gained and by the end of the day, the Austrian army was fatigued.