After his father's final exile, Berwick served in the French army under Marshal Luxemburg.
He fought at the battles of Steenkerque.
Luxemburg fell into William's trap set against superior numbers, but reinforcements failed the English, and the French rallied to send the Maison Militaire du Roi infantry downhill.
Berwick was in the charge of the division that broke the English lines.
They were driven from the field with heavy losses.
Berwick was one of Luxemburg's principal officers, and in 1694 commanded the centre of a large French army.
 After several forced marches to entrap William, they crossed the Meuse again before stopping them near Neerwinden.
Berwick struggled against the Foot Guards, who forced his men to retreat in the Landen.
He was taken prisoner by his cousin, Charles Churchill, and ransomed for 30,000 florins; and later was exchanged for a wounded Duke of Ormonde.
The Irish peer was accused of allowing himself to fall into enemy hands.
The scandal swept Whitehall Palace causing angry questions in Parliament.

At the same time a huge number of 400 ships bound for the Smyrna Convoy were taken by the French.
Because of his support for his father and service in the French army against England, he was attainted in 1695 by Act of Parliament rendering his British peerages forfeit.

As a soldier, Berwick was highly esteemed for his courage, abilities and integrity.
But when Marlborough challenged the French to fight at Liege, Boufflers retreated.
In June 1704, Berwick commanded a combined Franco-Spanish army but they did not seriously challenge the enemy, only taking a few of the Barrier Fortresses.
 By July 1706 Berwick had established increasing dominance in the north of Spain as the Bourbons' premier general.
In August partizans forced the Earl of Galway to evacuate Madrid allowing Berwick into winter quarters.
As a result of distinguished service in the War of the Spanish Succession, he became a French subject and was appointed a Marshal of France after his successful expedition against Nice in 1706.

Louis XIV was in the habit of sending his marshals lengthy orders.
In 1707 he wrote the duc de Vendome: "...it will be absolutely necessary for the Duke of Berwick to detach a similar proportion and to send a sufficient number of troops to the Elector of Bavaria..." The French allocated 75 battalions and 140 squadrons to the Rhine front, giving Berwick a comfortable numerical superiority.
On 25 April 1707, Berwick won a great and decisive victory at the Almanza, where an Englishman at the head of a Franco-Spanish army defeated Ruvigny, a Frenchman at the head of an Anglo-Portuguese-Dutch army.
Five English, six Dutch, and three Portuguese regiments were captured with all the cannon and regimental colours.
 The defeat ended Allied hopes of putting the Habsburg candidate on the Spanish throne.
 Berwick was "the brave English general who had defeated the French" to confused Tories.
After Almanza, Berwick was created Duque de Liria y Xérica (English: Duke of Liria and Jérica) and Lieutenant of Aragon by Philip V of Spain in 1707, and Duc de Fitz-James (English: Duke of Fitz-James) in the Peerage of France by Louis XIV in 1710.
 But in the meantime in July 1707, Berwick heard the Allies had left Maastricht and sent 34 battalions and 75 squadrons north to meet Prince Eugene's army.
In the spring of 1708, a Dutch force raided Berwick's camp at Cenco, killing all the men in their tents, with Berwick expressing shock at the "barbarous" action.
 By July Berwick had caught up with Vendome's army at Oudenarde, but was yet to arrive, anticipating Vendome would retreat to join up his force at Gavre six miles away.
But Berwick was at Givet on River Meuse when he heard of Vendome's heavy defeat.
Berwick rescued 9,000 stragglers using them to garrison Douai, and then moved Lille, where he prepared to be besieged.
However, the English arrived there first and managed to dismantle the defensive ramparts before he could reach them.
 Nonetheless on 3 August, Berwick reckoned a siege was afoot reinforcing the garrison with 20 battalions
After his father's final exile, Berwick served in the French army under Marshal Luxemburg. He fought at the battles of Steenkerque. Luxemburg fell into William's trap set against superior numbers, but reinforcements failed the English, and the French rallied to send the Maison Militaire du Roi infantry downhill. Berwick was in the charge of the division that broke the English lines. They were driven from the field with heavy losses. Berwick was one of Luxemburg's principal officers, and in 1694 commanded the centre of a large French army.  After several forced marches to entrap William, they crossed the Meuse again before stopping them near Neerwinden. Berwick struggled against the Foot Guards, who forced his men to retreat in the Landen. He was taken prisoner by his cousin, Charles Churchill, and ransomed for 30,000 florins; and later was exchanged for a wounded Duke of Ormonde. The Irish peer was accused of allowing himself to fall into enemy hands. The scandal swept Whitehall Palace causing angry questions in Parliament.
At the same time a huge number of 400 ships bound for the Smyrna Convoy were taken by the French. Because of his support for his father and service in the French army against England, he was attainted in 1695 by Act of Parliament rendering his British peerages forfeit.
As a soldier, Berwick was highly esteemed for his courage, abilities and integrity. But when Marlborough challenged the French to fight at Liege, Boufflers retreated. In June 1704, Berwick commanded a combined Franco-Spanish army but they did not seriously challenge the enemy, only taking a few of the Barrier Fortresses.  By July 1706 Berwick had established increasing dominance in the north of Spain as the Bourbons' premier general. In August partizans forced the Earl of Galway to evacuate Madrid allowing Berwick into winter quarters. As a result of distinguished service in the War of the Spanish Succession, he became a French subject and was appointed a Marshal of France after his successful expedition against Nice in 1706.
Louis XIV was in the habit of sending his marshals lengthy orders. In 1707 he wrote the duc de Vendome: "...it will be absolutely necessary for the Duke of Berwick to detach a similar proportion and to send a sufficient number of troops to the Elector of Bavaria..." The French allocated 75 battalions and 140 squadrons to the Rhine front, giving Berwick a comfortable numerical superiority. On 25 April 1707, Berwick won a great and decisive victory at the Almanza, where an Englishman at the head of a Franco-Spanish army defeated Ruvigny, a Frenchman at the head of an Anglo-Portuguese-Dutch army. Five English, six Dutch, and three Portuguese regiments were captured with all the cannon and regimental colours.  The defeat ended Allied hopes of putting the Habsburg candidate on the Spanish throne.  Berwick was "the brave English general who had defeated the French" to confused Tories. After Almanza, Berwick was created Duque de Liria y Xérica (English: Duke of Liria and Jérica) and Lieutenant of Aragon by Philip V of Spain in 1707, and Duc de Fitz-James (English: Duke of Fitz-James) in the Peerage of France by Louis XIV in 1710.  But in the meantime in July 1707, Berwick heard the Allies had left Maastricht and sent 34 battalions and 75 squadrons north to meet Prince Eugene's army. In the spring of 1708, a Dutch force raided Berwick's camp at Cenco, killing all the men in their tents, with Berwick expressing shock at the "barbarous" action.  By July Berwick had caught up with Vendome's army at Oudenarde, but was yet to arrive, anticipating Vendome would retreat to join up his force at Gavre six miles away. But Berwick was at Givet on River Meuse when he heard of Vendome's heavy defeat. Berwick rescued 9,000 stragglers using them to garrison Douai, and then moved Lille, where he prepared to be besieged. However, the English arrived there first and managed to dismantle the defensive ramparts before he could reach them.  Nonetheless on 3 August, Berwick reckoned a siege was afoot reinforcing the garrison with 20 battalions 
James FitzJames, 1st Duke of Berwick