The siege was the eastern end of a long collapse. On 26 June 1794, the French Revolutionary Army of Sambre-et-Meuse under General Jean-Baptiste Jourdan beat the Austrian army of the Prince of Coburg at Fleurus. The Coalition forces — Austrians, British, Dutch, with Hessian and Hanoverian contingents — began a retreat across what would soon stop being the Austrian Netherlands. Brussels fell on 10 July, then Leuven, then Mechelen. The Duke of York pulled the British and Hanoverians north into the Dutch Republic. By late July, French generals named Kléber and Hatry had taken Tongeren and Liège. The Austrian line ran north along the Meuse, with its centre at the fortress city of Maastricht — Vauban's old prize from 1673, garrisoned now by eight thousand Austrian and Dutch troops under Prince Frederick of Hesse-Kassel.

On General Kléber's staff in that autumn of 1794 was a hussar captain named Michel Ney, twenty-five years old, the son of a Saarlouis barrel-cooper. He had enlisted as a private nine years earlier. Kléber had spotted him and promoted him to Adjutant General on 1 August. At the start of the siege, Jourdan tried to keep Ney with the main army because he valued his abilities for cavalry raids — Ney was riding as far as Kleve and Nijmegen, screening the French right flank. But Kléber wanted him at Maastricht and got the Représentant en mission, Pierre-Mathurin Gillet, to reassign him. When Kléber summoned Prince Frederick to surrender on 1 November and the prince refused, eighty French siege guns opened fire. Within days the Austrian defenders' morale broke. Twice Kléber sent Ney into the fortress to negotiate. On Ney's second visit, he told Prince Frederick to his face that he was 'sacrificing the town to his personal glory'. The prince capitulated on 4 November. Ney would go on to become one of Napoleon's most famous marshals, the bravest of the brave, executed for treason in 1815. The negotiating skill he showed in this small room in Maastricht was the same one Napoleon would later rely on at Elchingen and Friedland.

Once the siege guns opened up, the Austrian troops inside Maastricht began doing what desperate garrisons have always done: they turned on the inhabitants. Austrian soldiers started plundering the homes of the city's people, breaking into shops and houses for food and goods, and the Dutch troops in the same garrison opened fire on them to stop it. A siege from outside became a small civil war inside. This was the context in which Ney walked through the breach for his second negotiation. The prince, whatever his personal glory, was now commanding a force that was destroying the city it was supposedly defending. Capitulation, when it came on 4 November, was a relief to almost everyone. The garrison marched out with the honours of war on 7 November and was allowed to go free. They surrendered 344 guns and thirty-one regimental colours. French casualties in the bombardment had been three hundred. Coalition losses were five hundred killed and wounded — light, by the standards of the place.

The fossil had been pulled from the marl quarries of Sint-Pietersberg in 1764 and again, more completely, in 1780. It had passed into the collection of a local canon named Godding, who refused to sell. After the siege, the French government bought, requisitioned or appropriated — accounts vary — the skull from his estate, with a colourful story attached that the troops were paid in wine to track down the right house. The skull went to Paris in 1795. There, Georges Cuvier examined it and argued, against still-prevailing assumptions, that it belonged to an animal that no longer existed anywhere on Earth. Cuvier did not name it; that would come in 1822, when the English geologist William Conybeare coined the name Mosasaurus — the Meuse lizard. The argument from this single skull, that whole species could go extinct, helped break open European natural history and clear ground for the work Darwin would do half a century later. The Netherlands has asked Paris, repeatedly, for the skull back. Paris has, repeatedly, declined.

The chronicler Phipps recorded an amusing scene from the siege that says something about how cities survive cannonade. A French mine detonated under one of the outer forts and exposed a cavern in the underlying marl. French soldiers rushed in, expecting Austrians — and found, instead, a herd of pigs that the garrison had stashed underground out of artillery reach. The marl caves under Maastricht and the surrounding hills had been used for hiding people, paintings, weapons and food for centuries by then. They would be used again 150 years later, when Rembrandt's Night Watch was stored in the Sint-Pietersberg during the Second World War. The pigs of 1794, like the masterpieces of 1944, were Maastricht's old answer to siege: when the surface is dangerous, go down. Even by 1794, after Vauban had drawn the geometry of attack and after eighty French siege guns had spoken, the cellars and caverns still held.

Maastricht's fall was not the end of the campaign. Jourdan pushed on, the Austrian army withdrew across the Rhine near Cologne on 6 October, and within a year the French had overrun the entire United Provinces and installed the Batavian Republic. The Austrian Netherlands ceased to exist; the territory was annexed into the French Republic itself, organised into new départements with new names. Maastricht became the capital of the département of the Lower Meuse. Twenty years of French rule followed, ending only when Napoleon fell in 1815 and the Congress of Vienna handed the region to the new Kingdom of the Netherlands. The Belgian Revolution of 1830 would carve it apart again. Each siege of Maastricht — 1579, 1673, 1794 — had been about who held the river crossing. Each had answered the question, for a generation or two, before the next army arrived. The skull in Paris has now outlasted all of them.