When the Grande Armée left the shores of the English Channel to fight in the War of the Third Coalition, Rey was given command of the Camp de Boulogne from 1805 to 1808.
In the latter year he was named a Baron of the Empire.
He served as chief of staff in Laurent Gouvion Saint-Cyr's VII Corps in Catalonia beginning in August 1808.
It is likely that he commanded a brigade at the Battle of Ocaña in November 1809.
One source stated that Rey led 3,500 men in six battalions.
It is probable that his brigade included three battalions each of the 12th Light and 43rd Line Infantry Regiments.
At the Battle of Baza on 4 November 1810, he led a brigade from Horace Sebastiani de la Porta's 1st Division of the IV Corps consisting of one battalion of the 32nd Line Infantry Regiment and three battalions of the 58th Line.
He fought at the Siege of Tarragona in 1811.
Historian David G. Chandler noted that Rey was appointed governor of the fortress of San Sebastián in August 1811.
Another source asserted that Rey was defeated by Francisco Ballesteros at Alhaurín el Grande on 14 April 1812.
On this occasion he led a force of 3,000 men made up of three squadrons of the 21st Dragoon Regiment, two artillery pieces, and three battalions divided between the 43rd and 58th Line Infantry Regiments.
French losses numbered 200 and included both guns.

From 5 July to 9 September 1813 Rey conducted an extremely able defense of San Sebastián.
In relating the story of the siege, Historian David Gates called Rey, "a commander of the first order."
The army of the Marquess of Wellington shipped 40 heavy siege cannon and large quantities of supplies to the nearby port of Pasajes in early July.
Wellington assigned Thomas Graham, 1st Baron Lynedoch the British 5th Division and a Portuguese brigade to undertake the siege operation.
An attempt to rush an outlying defense, the San Bartholomé Monastery, failed on 15 July.
A heavier bombardment and attack pried the French defenders from the monastery two days later, but the Allied infantry gave chase and were bloodily repulsed with 200 casualties at the main defenses.
The Anglo-Portuguese siege guns went to work and opened a small breach on the east side.
Meanwhile, the British engineers discovered an old sewer which they utilized to place a mine under the southern defenses.

Rey sealed off the breach with internal defenses, manned the nearby buildings with marksmen, and zeroed in his artillery on the threatened area.
At dawn on 25 July, Graham's attack went forward.
The mine exploded, doing some damage, but the French easily drove off the southern Allied attack, which was only a diversion.
Led by the Royal Scots, the main column splashed across the Rio Urumea estuary at low tide and assaulted the breach from the east.
After the attackers reached the top of the breach, the French opened fire and the result was a massacre.
The stunned British infantry stampeded, carrying away their supporting battalions.
The attackers suffered a loss of 600 killed and wounded, while French casualties were only one-tenth as great.
When the incoming tide threatened to drown the many British wounded, Rey generously allowed his soldiers to leave their defenses and rescue them.
The onset of the Battle of the Pyrenees that day caused Wellington to order Graham to abandon the siege.
As the Allies withdrew their siege guns, Rey launched a surprise sortie that inflicted 200 more casualties and seized some equipment.
Shaken by their setbacks, Allied morale dropped and desertion became a problem.

During the interlude, Rey repaired his southern defenses and thoroughly sealed off the breach with a new wall.
He also shipped in fresh troops and supplies through the leaky British naval blockade and sent his sick and wounded soldiers back to France.
Rey awaited a return of the Allies with 3,000 infantry and 60 cannon.
After defeating the French in the Pyrenees, Graham rearmed his siege batteries, which now numbered 63 heavy guns.
On 26 August the bombardment began again and lasted for five days.
The Allied artillery knocked out gun after gun and infli
When the Grande Armée left the shores of the English Channel to fight in the War of the Third Coalition, Rey was given command of the Camp de Boulogne from 1805 to 1808. In the latter year he was named a Baron of the Empire. He served as chief of staff in Laurent Gouvion Saint-Cyr's VII Corps in Catalonia beginning in August 1808. It is likely that he commanded a brigade at the Battle of Ocaña in November 1809. One source stated that Rey led 3,500 men in six battalions. It is probable that his brigade included three battalions each of the 12th Light and 43rd Line Infantry Regiments. At the Battle of Baza on 4 November 1810, he led a brigade from Horace Sebastiani de la Porta's 1st Division of the IV Corps consisting of one battalion of the 32nd Line Infantry Regiment and three battalions of the 58th Line. He fought at the Siege of Tarragona in 1811. Historian David G. Chandler noted that Rey was appointed governor of the fortress of San Sebastián in August 1811. Another source asserted that Rey was defeated by Francisco Ballesteros at Alhaurín el Grande on 14 April 1812. On this occasion he led a force of 3,000 men made up of three squadrons of the 21st Dragoon Regiment, two artillery pieces, and three battalions divided between the 43rd and 58th Line Infantry Regiments. French losses numbered 200 and included both guns.
From 5 July to 9 September 1813 Rey conducted an extremely able defense of San Sebastián. In relating the story of the siege, Historian David Gates called Rey, "a commander of the first order." The army of the Marquess of Wellington shipped 40 heavy siege cannon and large quantities of supplies to the nearby port of Pasajes in early July. Wellington assigned Thomas Graham, 1st Baron Lynedoch the British 5th Division and a Portuguese brigade to undertake the siege operation. An attempt to rush an outlying defense, the San Bartholomé Monastery, failed on 15 July. A heavier bombardment and attack pried the French defenders from the monastery two days later, but the Allied infantry gave chase and were bloodily repulsed with 200 casualties at the main defenses. The Anglo-Portuguese siege guns went to work and opened a small breach on the east side. Meanwhile, the British engineers discovered an old sewer which they utilized to place a mine under the southern defenses.
Rey sealed off the breach with internal defenses, manned the nearby buildings with marksmen, and zeroed in his artillery on the threatened area. At dawn on 25 July, Graham's attack went forward. The mine exploded, doing some damage, but the French easily drove off the southern Allied attack, which was only a diversion. Led by the Royal Scots, the main column splashed across the Rio Urumea estuary at low tide and assaulted the breach from the east. After the attackers reached the top of the breach, the French opened fire and the result was a massacre. The stunned British infantry stampeded, carrying away their supporting battalions. The attackers suffered a loss of 600 killed and wounded, while French casualties were only one-tenth as great. When the incoming tide threatened to drown the many British wounded, Rey generously allowed his soldiers to leave their defenses and rescue them. The onset of the Battle of the Pyrenees that day caused Wellington to order Graham to abandon the siege. As the Allies withdrew their siege guns, Rey launched a surprise sortie that inflicted 200 more casualties and seized some equipment. Shaken by their setbacks, Allied morale dropped and desertion became a problem.
During the interlude, Rey repaired his southern defenses and thoroughly sealed off the breach with a new wall. He also shipped in fresh troops and supplies through the leaky British naval blockade and sent his sick and wounded soldiers back to France. Rey awaited a return of the Allies with 3,000 infantry and 60 cannon. After defeating the French in the Pyrenees, Graham rearmed his siege batteries, which now numbered 63 heavy guns. On 26 August the bombardment began again and lasted for five days. The Allied artillery knocked out gun after gun and infli
Louis Emmanuel Rey