The battle of Casteggio-Montebello (9 June 1800) was a hard fought French victory that came as the main French and Austrian armies in Piedmont were closing in on each other in the build-up to the battle of Marengo. Until 8 June Napoleon had been hoping to force the Austrians, under Melas, to fight their way out of Piedmont by blocking the road along the Po at Stradella, but on that day Napoleon learnt that Genoa had fallen to the Austrians. This meant that Melas no longer needed to stay in Piedmont, and could bypass Napoleon via Genoa or Milan.
To avoid this Napoleon ordered his advance guard, under General Lannes, to advance west from Stradella towards Voghera, to clear up any Austrian outposts and in preparation for a move west by the entire army. Lannes had 12,500 men under his command, half in Watrin's division and half in Victor's.
Unknown to the French the Austrian army that had been besieging Genoa, under Karl Peter Ott Freiherr von Batorkez, was approaching the same place from the opposite direction, having marched north from Genoa to Novi, and from their towards Vogherra, with orders to join up with a second Austrian force under O'Reilly, which was retreating west from Piacenza. The Austrians thus had at least 14,500 men around Vogherra, 3,500 from O'Reilly's advance guard and 11,000 under Ott (some sources give them as many as 17,000 men).
Neither side appreciated the other's true strength at the start of the battle. The French didn't realise that Ott had reached the theatre of war from Genoa, while the Austrians believed that they were facing a small French force. This belief was encouraged by the first clash of the day, when General Gency and the 6th Légère clashed with O'Reilly at Santa Guilietta. The French retreated after a brief fight, and O'Reilly reported that he was facing 6,000 men.
At 9am the French returned to the attack, in somewhat larger numbers. O'Reilly was forced to retreat from Santa Giuletta to a village identified at Rivalta or Romero (probably Rivetta, two miles east of Casteggio). O'Reilly was able to fight off an attack by the 6th Légère, but at 11am Watrin arrived with more of his division, and decided to outflank the Austrians. Four battalions advanced south into the higher ground south of the road from Santa Giuletta to Casteggio, and two moved north onto the flatter ground between the road and the Po. O'Reilly held on for half an hour, but was then forced back by superior numbers.
The town of Casteggio was built at the point where a small stream flowed into the Coppa, a small river. The road entered the village across a bridge over the stream, turned right in a market square, and then crossed a second bridge over the Coppa. The village itself continued on to the south. As at Rivalta, the land to the north of the village was flat, that to the south was hilly.
At about noon Ott arrived at Casteggio, and decided to stand his ground. General Gottesheim was sent to occupy the hills to the south-east of the town, which dominated the main road, while O'Reilly with some of Ott's men defended the town. A reserve of eleven battalions was left at Montebello, two miles to the west.
Watrin followed O'Reilly towards Casteggio, where his advance was stopped by Austrian fire from the hills. Realising that Gottesheim's right flank was exposed Watrin decided to attack into the hills, sending one battalion from the 6th Légère and three from the 40th demi-brigade to carry out the attack. At the same time Lannes sent one battalion from the 22nd demi-brigade to make a frontal assault on the hills, while the two remaining battalions of the 6th Légère (under Gency) were sent north to attack the Austrian left flank at Casteggio.
Both French attacks began well. Although the frontal assault on the Austrian right was repulsed General Mahler was able to reach the crest of the hills, supported by General Mainoni, who had just arrived with the 28th demi-brigade. The Austrian right was driven back across the high ground to the south-west of Mairano, towards the smaller stream. In the village the 12th Hussars managed to cross the first bridge, but were then surrounded and forced to cut their way out.
Ott realised that his right wing was in danger, and called up six battalions from his reserve. IR 18 was used to support the line, and Gottesheim was able to return to the right, breaking one battalion from the 40th demi-brigade. Lannes sent the two remaining battalions of the 22nd to support the 40th, but before they could arrive Gottesheim and Vogelsang had reoccupied the heights.
Back in the village O'Reilly's light troops were engaged in a fight with the 6th Légère, and were coming under increasing pressure. The pressure was lifted by the arrival of Schellenberg's reserves (IR 28 and IR 40), which were sent to the line of the Coppa below Castegio to attack Gency.
The real crisis for the French came on their left, where only the 28th demi-brigade (Mainoni) was preventing the Austrians from reaching the road and cutting Lannes off from reinforcements. Fortunately for the French the 28th was able to hold out for long enough to allow Victor to arrive with the 24th Légère (Herbin) and the 43rd and 96th demi-brigades (Rivaud).
Victor's men were used to support new attacks on the Austrian right and centre. The attack on the right was made by Rivaud's 43th demi-brigade, which attacked with the central battalion in column and the two flank battalions deployed as skirmishers. The Austrians were forced back off the heights, but were able to retreat in good order, and formed a new line along the stream running south from Casteggio, centred on the Casa il Giardiana, a strongly build farm house.
Rivaud realised that this was the key to the Austrian line, and attacked the Casa with two companies from the 43rd demi-brigade. The Austrians were forced to retreat, and a gap was formed between Vogelsang and the right on the hills to the south and O'Reilly in Casteggio.
Ott now realised that he was facing the main French Army of the Reserve, and decided it was time to retreat. He hoped to hold the line of the Coppa, and so abandoned the eastern part of Casteggio and the first bridge. When the 24th Légère made the next French attack they reached the centre of the village with ease, but were then cut down by canister fire from Austrian guns protecting the bridge over the Coppa. The Austrian cavalry then surrounded the survivors, who were forced to fight their way out or surrender.
Lannes turned his attention to the Austrian left. The 6th Légère was moved north, crossing the Coppa on a minor bridge. Ott sent IR 40 north in an attempt to stop this movement, and also called up his last reserves, IR 51 and 13, from Montebello, but he was unable to stop the French advance. The 6th Légère almost surrounded IR 40, and IR 51 was forced to retreat back towards Montebello. The Austrian right was also making its way back to this village, where Ott hoped to form a new defensive line. O'Reilly was left behind to form a rear guard in Casteggio, losing 400 men taken prisoner.
The French attacked the new Austrian line at about 8pm. This time Schellenberg formed the Austrian rear-guard, and he was able to hold the French up for long enough for the rest of Ott's men to reach Voghera, another six miles to the west. Schellenberg's men fought until darkness fell, and then made their own escape.
The Austrians suffered heavy losses at Casteggio-Montebello, taking 2,100 casualties and 2,500 prisoners, one third of the entire force engaged. Lannes suffered similar losses.
In the aftermath of this defeat Ott retreated west back towards Alessandria, where he found Melas with Kaim, Hadik and the army from Turin. The French followed, now aware that they were facing the main Austrian army in Piedmont. The final clash of the campaign would come five days later, at Marengo.