In 1809 he became chief of staff to Marshal André Masséna but ran afoul of Emperor Napoleon and was banished from the army for several years.

After Napoleon's defeat at the Battle of Waterloo and the collapse of his regime, the interim French government appointed Beker to guard the ex-emperor.

On the night of 16 September 1796 he notified the commander of the right flank division, Jean Castelbert de Castelverd that the Austrians had won a crossing of the Lahn River at Diez but that French forces had sealed off the bridgehead.

[2] During the War of the Third Coalition, Beker commanded an infantry brigade in Louis Gabriel Suchet's division in Marshal Jean Lannes' V Corps.

[14] His independent command later became known as the 5th Dragoon Division and served first under Anne Jean Marie René Savary and later under Marshal André Masséna near Warsaw.

[20] Historian Francis Loraine Petre wrote that Masséna had grown lazy and delegated too many responsibilities to Beker, "probably also a great deal of the framing of orders".

[22] According to Petre, Napoleon took notice of the marshal's habit of letting Beker run the corps for him and removed the chief of staff from his post, "probably unjustly".

Within weeks, Napoleon was decisively defeated at the Battle of Waterloo and on 20 June Beker was ordered to report to Paul Grenier and assist in the defense of Paris.

[17] At the orders of Marshal Louis-Nicolas Davout, Beker was sent to the Château de Malmaison purportedly "to ensure the Emperor's safety".

Actually, Joseph Fouché, who controlled the interim government, feared that Napoleon might rejoin the French army and continue the war.

Fouché and Davout knew there was every reason for Beker to dislike Napoleon and found him the perfect person to keep a close guard on the abdicated emperor.

[17] Napoleon hoped to flee to the United States from the port of Rochefort, while Fouché wished to hand the ex-emperor over to the Allies.

[28] After the event, Louis XVIII's war minister Marshal Laurent Gouvion Saint-Cyr peremptorily ordered Beker to go home and remain there.