Coucy fought in the Battle of Nicopolis in 1396 as part of a failed crusade against the Ottoman Empire, but was taken prisoner and contracted the bubonic plague.

Coucy first became involved in the war against England at the age of fifteen, serving among the barons of Picardy in the battalion of Moreau de Fiennes.

He was retained as a hostage in 1360, when the Treaty of Brétigny established territorial adjustments between the two countries, and set the monetary payments for King John's release.

He was in great favour with both the French and English..." In 1365, the wealthy Coucy was betrothed and married to the 33-year-old Isabella of England, who has been described as an over-indulged, willful, and wildly extravagant princess.

To care for her personal needs, her father settled a substantial annual income on her for life, as well as gifts of costly jewellery, and properties that included manors, castles, and priories.

Coucy was her choice as a husband, as she wished to marry for love after the failure of previous betrothal negotiations with several noble houses of Europe.

At this time, Coucy was presented with additional French lands, receiving the title Count of Soissons, which had come to King Edward III through the payment of ransom.

In the autumn of 1375 Coucy engaged a number of Free Companies, including one led by Owain Lawgoch, to seize some Habsburg lands which he claimed through his mother.

Eager for glory, these knights then led their forces in a reckless pursuit of the fleeing Turks, only to run up against a fresh corps of Turkish sipahis that Bayezid had kept in reserve.

A desperate battle ensued, but at the height of the fighting Bayezid's Serbian ally arrived with reinforcements, turning the tide in the Turks' favour.

Coucy and many other leading nobles were taken prisoner, and the next day Bayezid forced the knights to watch the day-long mass beheading of hundreds (and possibly as many as 3000) Crusader soldiers who had been captured by the Turks.

Although strenuous efforts were made in France over the next few months to arrange the release of the captives, Coucy died before his bounty could be paid, due to an outbreak of the bubonic plague among the Turks, although it is likely that he had already been greatly weakened by the wounds he suffered at Nicopolis, and the hardships of the subsequent forced march.

Coucy was responsible for the maintenance of the castle and additional construction on his familial estates, which consisted of the fortress, 150 towns and villages, famous forests and ponds, along with significant revenue.

[citation needed] The courtship of Coucy and his first wife comprises a large portion of the plot of The Lady Royal, a novel by Molly Costain Haycraft.