Battle of the War of the Second Coalition
The Marengo campaign (4 April – 15 June 1800) saw a Habsburg Austrian army led by General der Kavallerie Michael von Melas fight against the defending French Army of Italy under General of Division (GD) André Massena and the invading French Reserve Army commanded by First Consul Napoleon Bonaparte. In early April, Melas launched a successful offensive that split the outnumbered Army of Italy and initiated the Siege of Genoa with Massena's forces trapped within the city. In mid-May, Bonaparte led the Reserve Army across the Great St Bernard Pass and into the Aosta Valley. After encountering a serious delay at Fort Bard, Bonaparte's forces broke into the plains of the Po Valley toward the end of May. At Turin, Melas blocked the direct route to Genoa, but Bonaparte's forces instead seized Milan and began severing the supply lines between Melas' army and Austria. After a long siege, Massena finally surrendered Genoa in early June, but by that time Melas' forces were isolated in northwest Italy. Melas tried to break out of the trap in the Battle of Marengo on 14 June 1800 and nearly succeeded because Bonaparte spread his army too thin. However, late-arriving troops defeated the Austrians and Bonaparte compelled Melas to evacuate northwest Italy as the price of a negotiated truce. 
Background: 1799.
In early 1799, the French Directory deployed two armies in Italy. The Army of Naples led by GD Étienne Macdonald occupied Rome and Naples with 32,010 men. The Army of Italy under GD Barthélemy Louis Joseph Schérer defended northern Italy. In March, the Directory ordered Schérer to detach a 6,400-man division to invade and plunder the Grand Duchy of Tuscany. This left Schérer with only 43,000 men in his field army, not counting various occupying forces. The Austrian army commanded by FML Paul Kray counted 50,700 men, but a 24,551-strong Russian army under Field Marshal Alexander Suvorov was approaching. The indecisive Battle of Verona on 26 March was followed by the Battle of Magnano on 5 April 1799. At Magnano, the victorious Austrians reported 5,228 casualties but French losses were heavier. Leaving 12,000 men to defend Mantua, Schérer started a panicky retreat that did not stop until the French army reached the Adda River. 
Schérer was replaced in command by GD Jean Victor Marie Moreau but the French were beaten at the Battle of Cassano on 27–28 April 1799. The string of major defeats continued with the Battle of Trebbia on 17–20 June, the surrender of Mantua on 28 July, the Battle of Novi on 15 August, and the Battle of Genola on 4 November. By the end of 1799, the French hold on Italy had dwindled to almost what it was in March 1796 when Bonaparte assumed command of the army. The badly fed and supplied French army clung to Genoa and the Italian Riviera. The army commander GD Jean-Étienne Championnet wished to abandon Genoa, but the Directory stubbornly refused to allow this. Championnet died of disease on 9 January 1800 and was eventually replaced by André Massena. 
On 9 October 1799, GD Napoleon Bonaparte returned to France from Egypt. The "weak and corrupt" French Directory was very unpopular. Bonaparte immediately entered into a conspiracy to overthrow the Directory. The Coup of 18 Brumaire (9 November 1799) was successful and on 25 December, Bonaparte became First Consul, a position in which he held dictatorial power. On 25 January 1800, Bonaparte ordered GD Louis-Alexandre Berthier to assemble a 60,000-man Reserve Army at Dijon. To trick enemy spies, some second-class units began arriving at Dijon, while the first-class units began massing elsewhere. The Austrian government was completely fooled by this ruse and did not realize that the Reserve Army represented a serious threat.
Strategy.
By 1800, the Russian Empire had withdrawn from the Second Coalition. The Austrians held Swabia in southern Germany and northern Italy. Since the French held Switzerland, the Austrian armies in Germany and Italy had difficulty communicating with each other. Meanwhile, the French could use Switzerland to easily shift forces from one theater to the other. Along the Rhine River, GD Jean Victor Marie Moreau led a 120,000-man French army, composed of the best troops available. Opposed to Moreau on the east bank of the Rhine was an Austrian army nearly as strong commanded by Paul Kray (now promoted to Feldzeugmeister). In Italy, Michael von Melas commanded an army of 100,000 men, including 86,000 infantry and 14,000 cavalry. Massena's Army of Italy was divided into three corps under GD Jean-de-Dieu Soult (19,790 men), GD Louis-Gabriel Suchet (15,607 men), and GD Louis Marie Turreau (8,000 men). It fielded only 36,000 men for active duty, though 14,000 sick soldiers were in hospitals. 
The Austrian Aulic Council planned for Melas to capture Genoa, cross the Var River, and lay siege to Toulon. This would cause Moreau to send reinforcements from his own army. With Moreau weakened, Kray would then push across the Rhine and invade France through the Belfort Gap. Caught between the two thrusts, Switzerland would easily fall to Austria, and then Melas would march up the Rhône valley. The Kingdom of Great Britain pledged to help Austria capture Genoa and to assemble an army at Minorca that would be used to help Austria invade Provence. At first, Bonaparte planned to use the Reserve Army to carry out an enveloping maneuver against Kray's army in Swabia. However, Moreau obstinately refused to cooperate with Bonaparte's strategy. Therefore, Bonaparte left Moreau to carry out his own operations and instead decided to commit the Reserve Army to Italy. Bonaparte arranged for Moreau to transfer GD Claude Lecourbe's 25,000-man corps from Germany via Switzerland to assist his Italian campaign. 
Austrian offensive.
On 4 April 1800, Melas mounted his offensive against the Army of Italy and Genoa. According to James R. Arnold, Melas ordered FML Peter Karl Ott von Bátorkéz and 8,000 troops to advance from Bobbio, northeast of Genoa, while FML Prince Friedrich Franz Xaver of Hohenzollern-Hechingen and 5,300 soldiers moved against the Bocchetta Pass north of Genoa. West of Genoa, Melas with 27,500 men struck south from Acqui Terme and FML Anton von Elsnitz thrust east from Ceva with 21,100 troops. While Melas' main army attacked Genoa with 62,000 men, FML Konrad Valentin von Kaim with 31,000 soldiers watched the mountain passes and northern Italy. Another 20,000 men garrisoned various strongpoints in Italy. Theodore Ayrault Dodge assigned different strengths to the attacking columns: 40,000 soldiers to Melas, 15,000 to Ott, and 10,000 to Hohenzollern. 
By 6 April, the columns of Melas and Elsnitz successfully broke the connection between the corps of Soult and Suchet at Cairo Montenotte. This breakthrough prompted Massena to mount a determined effort to reconnect with Suchet's forces. Massena left 8,000 troops in Genoa and tried to cut his way out, resulting in ten days of brutal fighting in the mountains. There were actions at Cadibona Pass on 6 April, Monte Fasce on 7 April, Bocchetta Pass on 9 April, Sassello on 10 April, Monte Settepani on 10–11 April, Vetriera on 11 April, Colle di San Giacomo on 12 April, and Voltri on 18 April. From 6 to 19 April, Austrian losses were 276 officers and 8,037 men, while the French lost around 7,000 casualties. The fighting resulted in Massena's troops being cooped up in Genoa. The Siege of Genoa would last from 19 April to 4 June 1800. The blockade was assisted by a British Royal Navy squadron under Admiral Lord Keith. 
On 24 April 1800, Melas demanded the surrender of Genoa which Massena refused. Melas assigned Ott and 24,000 soldiers to carry out the siege while accompanying Elsnitz and 30,000 troops to a pursuit of Suchet's corps along the coast. (Dodge credited 28,000 soldiers to Elsnitz.) On 30 April, Ott attempted to storm the defenses of Genoa but was repulsed with 3,147 casualties. The French defenders sustained 1,526 casualties, but the assault nearly succeeded. After this failure, Ott decided to starve the defenders into surrendering. Massena launched several sorties, including one led by Soult on 11 May that inflicted losses of 137 killed, 328 wounded, and 1,362 captured on the besiegers. Two days later, a French sortie failed, and Soult was wounded and taken prisoner. 
While Genoa was under siege, Elsnitz's corps drove Suchet's weak forces to the west along the Mediterranean coast, taking Albenga on 3 May. Suchet's rearguard held its position too long on 7 May at Montecalvo and was trapped, losing 1,500 prisoners. The Savona fortress capitulated to the Austrians on 15 May. Melas occupied Nice on 11 May. Elsnitz found Suchet's troops in defensive positions along the Var River. On 20 May, the British navy landed heavy artillery to support the Austrian advance. Meanwhile, Melas began receiving disturbing reports about French activities in Switzerland. As early as 8 May, Melas transferred one cavalry and two infantry regiments from Elsnitz to Kaim's forces in the north. On 13 May, Melas left Nice; he took 9,000 troops with him and set out for Turin. From 22 to 27 May, Elsnitz would confront Suchet's defenses along the Var.
French invasion.
Sometime in March 1800, as Bonaparte looked over a large map of Piedmont, he asked his secretary Louis Antoine Fauvelet de Bourrienne, "Where shall we beat the Austrians?" When Bourrienne replied that he didn't know, Bonaparte responded by asserting, "Why you idiot, don't you see that as Melas is at Alessandria, and has there his magazines and material, if I cross the Alps at the Great St Bernard I shall cut his communications, and beat him at San Giuliano?" Bourrienne recalled this conversation three months later. San Giuliano is only east of Marengo. After sending word to Massena to hold out until at least 4 June, Bonaparte left Paris and reached Geneva on 8 May. 
Berthier was the nominal Commander-in-chief of the Reserve Army and GD Pierre Dupont was its Chief of staff. The army's Chief of Artillery was General of Brigade (GB) Auguste de Marmont who commanded 48 guns. GD Jean Lannes' vanguard included GD François Watrin's division, a brigade led by GB Joseph Mainoni, and a cavalry brigade led by GB Jean Rivaud. The organization of the Reserve Army on 10 May 1800 was as follows. 
On 25 April 1800, Moreau finally began his operations against Kray's army. In early May, Moreau was victorious at the battles of Stockach, Messkirch, and Biberach, forcing Kray to retreat to Ulm. However, Moreau became reluctant to release Lecourbe's corps. On 13 May 1800, Moreau notified Bonaparte that he could only spare GD Bon-Adrien Jeannot de Moncey leading a smaller corps. Ultimately, Moncey brought 11,500 soldiers into northern Italy via the Gotthard Pass. Aside from Bonaparte's army and Moncey's corps, the French used three other passes in order to confuse the Austrians. Chabran's division utilized the Little St Bernard Pass, Turreau's forces crossed the Mont Cenis Pass, and one demi-brigade used the Simplon Pass. 
To oppose the French, Melas had 8,000 troops under FML Josef Philipp Vukassovich at Bellinzona observing the Gotthard Pass, 3,000 men in the Aosta Valley under Generalmajor (GM) Auguste-François Landres de Briey, 5,000 soldiers watching Mont Cenis Pass under Kaim, 1,000 men observing the Stura di Lanzo, 10,000 troops in garrisons in upper Italy, 3,000 soldiers in Tuscany, 3,000 men in Istria, and 6,000 troops en route. The crossing of the Great St Bernard Pass began early on 15 May when Lannes' vanguard set out. The cannons were dragged over the pass in hollowed-out tree trunks. Lannes reached Aosta on 16 May, drove out its small Austrian garrison, and was joined by Chabran's division the following day. Each subsequent division crossed at daily intervals. After Lannes and Chabran, the order of march was Boudet, Loison, Chambarlhac, and Monnier. 
By 17 May, Lannes had 12,300 infantry, 1,400 cavalry, and 14 artillery pieces under his command in the Aosta Valley. On 18 May there was a clash at Châtillon where Lannes' men captured 350 Austrians and 2 cannons from Briey's brigade at the cost of 90 casualties. On 19 May, the French advance came to a sudden halt where Fort Bard overlooked the main road in a narrow valley. Though Captain Stockard di Bernkopf commanded only 350 soldiers, the fort bristled with 16 large and 26 medium caliber cannons, plus a number of lighter guns. A bombardment on