Bernhard Erasmus von Deroy (11 December 1743 – 23 August 1812) from the Electorate of the Palatinate became a noted general officer in the army of Bavaria.

Deroy and his colleague, Karl Philipp von Wrede, were dominant personalities in the Bavarian military during the era of Napoleon Bonaparte.

Born in Mannheim on 11 December 1743, Bernhard Erasmus von Deroy's parents were General Matthias Bertram de Roy and Elizabeth Christine of Hofstatt.

He was appointed ensign in the Count Palatine Karl August Infantry Regiment at a very young age and fought in the Battle of Hastenbeck on 27 June 1757.

[1] The War of the Second Coalition broke out in 1799 and in the spring of 1800, Lieutenant General Christian Zweibrücken appointed Deroy to lead the 6,000-strong 1st Brigade as part of a German auxiliary corps allied with the Austrians.

[5] In Zweibrücken's Bavarian Division, Deroy led the Reuss Grenadiers, Metzen Light, Schlossburg, Stengel, and Minucci infantry battalions.

[6] In the Battle of Hohenlinden on 3 December 1800, the Bavarians marched with the Left Center Column, which was under the command of Johann Kollowrat and accompanied by Archduke John.

However, the allies soon ran into Emmanuel Grouchy's main line of resistance and were driven back by a powerful counterattack.

After heavy fighting, Kollowrat's column was caught in a three-sided trap between the divisions of Richepanse, Grouchy, and Michel Ney.

[8] In 1801 Elector Maximilian IV Joseph of Bavaria named Deroy to a commission to improve and reorganize the army.

When the War of the Third Coalition broke out in September 1805, the Bavarian army withdrew north to the Main River in response to the Austrian invasion.

The position was defended by Franz Xaver Saint-Julien's 1,500-man Austrian brigade with the support of local Tyrolean militia and sharpshooters.

Prince Jerome's 22,000-strong IX Corps maintained the siege until 6 January 1807 when the fortress capitulated with 5,300 prisoners and 268 cannon.

He moved against Srebrna Góra (Silberburg) with 6,500 troops, but the Prussian garrison refused to give up after he bombarded the fortress on 28 and 29 June.

This unit consisted of the 5th, 9th, 10th, and 14th Infantry Regiments, the 5th and 7th Light Battalions, the Taxis Dragoons and Bubenhoven Chevau-légers, and 18 pieces of artillery.

[15] After the Austrian invasion, Deroy held the west bank of the Isar River opposite Landshut on 16 April 1809.

[16] Under the command of Marshal François Joseph Lefebvre, Deroy was present but not engaged in the Battle of Abensberg on 20 April.

[17] On 21 April, as Napoleon was winning the Battle of Landshut to the south, Deroy fought against Prince Franz Seraph of Rosenberg-Orsini's Austrian IV Armeekorps.

[19] In the Battle of Eckmühl on 22 April, Davout and Deroy advanced from the west, while Napoleon fell on Rosenberg's position from the south.

[22] After Eckmühl and the minor Battle of Neumarkt-Sankt Veit on 24 April, Napoleon urged Lefebvre to seize Salzburg and relieve the Bavarian garrison of the Kufstein Fortress.

[23] In early May, Austrians and Tyrolean irregulars defeated General-Major Vincenti's brigade of the 3rd Division in its attempt to reach Kufstein.

[25] With Wrede's and Deroy's divisions, Lefebvre routed Johann Gabriel Chasteler de Courcelles at the Battle of Wörgl on 13 May.

[1] When Napoleon invaded the Russian Empire in 1812, Deroy commanded the 19th Infantry Division in Laurent de Gouvion Saint-Cyr's VI Corps.

[33] The march into Russia was difficult and the Bavarian general reported to his king that the food was bad and the soldiers' uniforms and shoes had worn out.

He then built a concealed bridge and secretly shifted his small army to the north bank on the night of 17–18 August.