King James made him a Knight of the Order of the Garter and appointed him Colonel of The Blues to replace the Protestant Earl of Oxford.

[5] The post had been coveted by his maternal uncle the Duke of Marlborough, but FitzJames was earmarked for command, as Catholics replaced Anglicans.

[9] When his father departed for France after the Boyne, Berwick remained with the Jacobite Irish Army during the retreat to Limerick.

On 2 August he was one of the Generals, with Patrick Sarsfield and Boisselau, who shored up the defences at Limerick awaiting the Williamite assault; thereafter they rode north across the Shannon with a few Guards.

Berwick arrived too late at the siege of Cork with 4,000 French troops, but unable to effect a result, he withdrew; Marlborough marched west to Kinsale to deal with 18,000 Frenchmen.

[12] After his father's final exile, Berwick served in the French army under Marshal Luxembourg, including at the battles of Steenkerque.

Luxembourg fell into William's trap set against superior numbers, but reinforcements failed the Allies, and the French rallied to send the Maison Militaire du Roi infantry downhill.

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.

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.

In August partizans forced the Earl of Galway to evacuate Madrid allowing Berwick into winter quarters.

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.

Nonetheless, on 3 August, Berwick reckoned a siege was afoot, reinforcing the garrison with 20 battalions and 7 squadrons of dragoons with horses.

Astutely, Berwick realized Marlborough was trying to bring his battered army to battle, despite constant missives from Versailles ordering an immediate attack.

Long before he had become a Marshal of France, he had remained in contact with his uncle's family when they were in exile, when the siege at Lille began.

[29] The last great event of the War of the Spanish Succession was the Duke of Berwick's storming of Barcelona, after a long siege, on 11 September 1714.

Trying to explain the failure of the Jacobite Risings, the Old Pretender "never forgave his half-brother, The Blues ex-colonel, Berwick, now an experienced and competent commander, for declining to lead his forces".

In December 1718 he led an army to Spain, against Philip V, in the War of the Quadruple Alliance, bombarding San Sebastián and occupying the districts of Gipuzkoa and Biscay.

[citation needed] They married in Saint-Germain-en-Laye (Yvelines) on 26 March 1695,[31] two years after Sarsfield's death in the Battle of Landen on 29 July 1693.