The alliance between England and Catholic France had been unpopular from the start and although the real terms of the Treaty of Dover remained secret, many suspected them. In early 1674, Denmark joined the Alliance, while England and the Dutch made peace in the Treaty of Westminster. William III tried to recapture Maastricht in 1676 but failed, the French having greatly improved the fortifications according to a plan drawn by de Vauban. Immediately after the siege, de Vauban spent three weeks surveying the city and its surroundings to create detailed maps of the terrain. Then a large-scale maquette of the fortress was made. After much controversy among historians, the present consensus is that this model was discarded relatively early and is not identical to the extant Maastricht maquette in the Paris Musée de l'Armée which shows the situation in the middle of the eighteenth century. On 16 September 1673, de Louvois had written that Louis would sooner give up Paris or Versailles than ever return Maastricht, but the city was nevertheless returned to the Dutch when the Treaty of Nijmegen ended the war in 1678. The French used the city as a bargaining chip to seduce them to cease supporting the Spanish war aims. The Dutch had promised that after a victory, the city would be ceded to the Spanish Netherlands, but refused when peace had been signed, claiming that otherwise the treaty conditions would be violated.