Short answers, pulled from the story.
The Battle of Marengo was fought on the 14th of June 1800 near the city of Alessandria in Piedmont, Italy. The fighting took place on a plain east of Alessandria, centered on the villages of Marengo, Castel Ceriolo, and San Giuliano Vecchio.
The French forces were led by Napoleon Bonaparte, serving as First Consul, with General Louis Alexandre Berthier as the official army commander. The Austrian forces were commanded by General Michael von Melas, who was 71 years old and slightly wounded during the battle before handing command to General Anton von Zach.
General Louis Desaix arrived with 6,000 men and 9 guns of Boudet's division at around 5:30 pm, after the French were already in retreat. His infantry stabilized the French line and drew the Austrian pursuit forward, allowing Kellermann's cavalry charge to break the Austrian formation. Desaix was shot from his horse at the decisive moment and did not survive the battle.
The Austrians lost approximately 6,500 dead or wounded, nearly 8,000 taken prisoner, 40 guns, and 15 colours. French casualties were on the order of 4,700 killed and wounded, with 900 missing or captured. General Desaix was among the French dead.
Napoleon commissioned three increasingly idealized official accounts of the battle during his reign. The army bulletin issued the day after the battle downplayed General Kellermann's decisive cavalry charge by pairing it with a separate action by General Bessières. Berthier's 1804 Relation de la bataille de Marengo reframed the French afternoon retreat as a deliberate strategic maneuver, contradicting eyewitness accounts from Marmont, Captain Coignet, Captain Gervais, and General Thévenet.
The Austrians fell back into Alessandria, having lost about half the forces they committed to the battle. Within 24 hours, General Melas entered into the Convention of Alessandria, which required the Austrians to evacuate northwestern Italy west of the river Ticino and to suspend military operations in Italy.