At the age of 19 in 1583, he travelled to Malta to perform his service as a Knight of the Order of Saint John, finding success in his campaign.

In 1589, after the assassination of his cousin by Henri III Aumale and the ligue at large entered open war with the king, who was forced into an alliance with the Protestant Navarre.

In the wake of the battle Navarre attempted to besiege Paris, with Aumale leading the defence alongside the duke of Nemours and the Seize.

[1] Destined for an ecclesiastical career, Aumale was granted several abbeys in commendam, bequeathed to him by his guardian uncle Cardinal Lorraine.

[6] Shortly after the assassination of the duke of Guise Rouen defected from the crown to the ligue, the royal militia commander Dyel had at first been complacent about the threat of a takeover, until he overheard one of his men bemoaning the fact that Aumale was not in the city to lead them.

[7] Henri III, now on the back foot, joined forces with the Protestant Navarre, forming a compact in April at Plessis-lès-Tours.

His advance guard, led by the chevalier d'Aumale met the king accompanied by only a small party on the north bank of the Loire, near Tours on 8 May.

The king appealed for support from elements of Navarre's army, under La Trémoille and Châtillon, who arrived late in the day to reinforce the bridge.

[13] Navarre's right, led by squadrons under Gaspard de Schomberg faced off against units under the command of the chevalier d'Aumale who had with him a Spanish Walloonian company.

[15] Navarre again made an attempt to capture Paris in 1590, but was again confounded, This time by the arrival of the army of the duke of Parma.

Prior to Parma's arrival, the defence of the city had been led by the duke of Nemours, and the chevalier d'Aumale who worked in an uneasy alliance with the Seize.

[16] Unwilling to fully cut his losses, Navarre decided to conduct a more limited objective of controlling the northern Parisian basin.

[17] Ligueur pamphleteers characterised him as a Christian warrior and a martyr for the holy cause, his loss was greatly mourned as he had been one of the ligues chief heroes.