One of the interesting aspects of Puccini's Tosca, is that much of the drama is driven by the historical events which surround the action. Prominent in the text is mention of the battle between the French and Austrian forces on the plain of Marengo.
During a daring campaign in 1796-97 the French regained control of northern Italy. In 1799, despite the brilliance of Napoleon's General Massena, he could not overcome the superior numbers, and when General Aleksandr Suvorov arrived with 18,000 Russian troops to support the Austrian army, Napoleon's forces were pushed out of northern Italy once again. Northern Italy was restored to Austria, and Massena retreated to the Rhine River.
In 1799, after Napoleon, then First Consul of France, left what remained of his Eastern forces in Egypt, he faced several obstacles. Chief among these were: the English had control over the Mediterranean Sea; they had effectively blockaded the Atlantic Coast of France; and there was a Russo-Austrian army in northern Italy. More importantly, Napoleon was threatened politically at home. Now that he possessed powers over war and peace, he needed to consolidate his grip on government, and gain a spectacular success in order to calm royalist sentiments in Paris and solidify his position as First Consul.
The fact that Napoleon's General in Germany and Italy, Massena, was able to retain control of Switzerland, gave the First Consul the route he needed to strike against the Austrians. While Massena massed his army to the east around Genoa, Napoleon was assembling a reserve force of approximately 50,000 troops just west of Switzerland
On May 1, Napoleon learned that Massena's forces had been split in two and were now facing a superior force of Austrians under the command of General Michael von Mélas. He sent Massena orders to hold his position in Genoa until May 30, and used the opportunity to begin crossing the mountains into Italy. Mélas continued his campaign against Messena's forces. He wanted to drive the French out of Italy, however, this left him completely unaware of Napoleon's descent upon him from the north.
Napoleon's army crossed the Alps through five separate passes in order to conceal the main body of his forces. As a result, Mélas was unable to identify the French forces as anything more than diversionary troops. On May 19, he learned the truth. The French were descending upon Italy in force! Mélas turned to meet the threat, but was faced with one foreboding question; which of the five approaching armies was the main French force? Against incredible difficulties, Napoleon was crossing with the main French column through the Great St. Bernard Pass.
Napoleon had several alternatives. Mélas had finally discovered his intentions and was in the process of concentrating his forces at Turin. Napoleon could move South to the aid of Massena, or attempt to take Milan, the main Austrian supply base. He chose the latter, and ordering General Lannes forces to the southeast in order to screen his movements, he headed East toward Milan, taking the city on June 2. By sending Lannes's to the south, however, Napoleon had divided his forces in the face of the enemy. This would prove to be a critical mistake which would almost cost him the campaign.
On June 4, Massena surrendered Genoa, and the Austrians moved north to find Lannes's army. Lannes forced the Austrians to retreat, cutting off all of Mélas' escape routes to the East. Mélas decided that he would have to face the French army near Alessandria and fight his way out. Napoleon, thinking that Mélas was still trying to elude him, further dispersed his forces in an attempt to find the Austrians. Not knowing that an overwhelming Austrian force lay just on the other side of the Bormida River, he placed himself in a disastrous position.
On June 14, Mélas crossed the Bormida River and attacked, deploying his forces on the Marengo plain. Three hours later, the French, under the command of General Claude Perrin Victor, retreated. Napoleon did not believe that he had met the main Austrian force, and ordered General Desaix, a veteran of Egypt and one of Napoleon's best fighters, to move further Southeast to find Mélas.
With the French on the run, and anxious to reassure the Emperor of a victory, Mélas sent out a number of dispatches saying the French had been defeated (in the opera this is the news that the Sacristan is telling the children's chorus in Act I and is the reason for the celebration at which Tosca is to sing).
Fortunately for Napoleon, Desaix knew that the Austrians were not waiting to the southeast, and as a result, he hesitated when he received Napoleon's orders to march further south. As he was finally preparing his troops to carry out the order, a messenger arrived from the First Consul, "Mélas has attacked me first. For God's sake come if you can!" By the time Desaix arrived, Napoleon had used all of his reserves and the French were on the retreat. Napoleon told Desaix, "The battle is lost." "Yes," Desaix replied, "But there is time to win another."
In a brilliant counterstroke, Desaix turned to meet Mélas head on, and within several hours turned defeat into victory. At the critical moment of the counterattack, Desaix was mortally wounded. Fearing that his death would destroy the morale of his troops, he gave orders that he was not to be moved from the field of battle. His troops were so outraged at the Austrians for having struck down their commander, that they found new courage and drove the Austrians from the field.
It is at this point in the opera, that Sciarrone arrives with the terrible news and informs Scarpia that what they had thought was a victory against Napoleon, was in fact, a defeat, and Cavaradossi convicts himself by shouting "Victory!".
Napoleon could not rejoice. He bore a deep personal loss in General Desaix. In addition, he had endangered the army by dividing his forces and almost lost the campaign. On June 15, Baron von Mélas asked Napoleon for truce terms. The battle had recovered Italy for the French, and stabilized the political climate at home for Napoleon. Marengo also marked the end for the Republic in France. Following the battle, Napoleon sent word to Paris saying, "I hope the French people will be pleased with their Army." Dictatorship was just over the horizon, and with it, greater desires for military glory.
Note: Tosca takes place on June 17, three days after the Battle of Marengo. It took that long for the dispatches to cover the approximately 250 miles from Marengo to Rome.