The Luxembourg Crisis (, ) was a diplomatic dispute and confrontation in 1867 between France and Prussia over the political status of Luxembourg. The confrontation almost led to war between the two parties, but was peacefully resolved by the Treaty of London. Background Luxembourg City boasted some of the most impressive fortifications in the world, the Fortress of Luxembourg, partly designed by Marshal Vauban and improved by subsequent engineers, which gave the city the nickname "Gibraltar of the North". Since the 1815 Congress of Vienna, the Grand Duchy of Luxembourg had been in personal union with the Kingdom of the Netherlands. In a concession to neighbouring Prussia, Luxembourg was to be a member of the German Confederation, with several thousand Prussian soldiers stationed there. The Belgian Revolution of 1830 had divided Luxembourg into two, threatening Dutch control of the remaining territory. As a result, William I of the Netherlands entered Luxembourg into the German customs union, the Zollverein, to dilute the French and Belgian cultural and economic influence in Luxembourg. Austro-Prussian War The Second Schleswig War of 1864 had further advanced nationalist tensions in Germany, and, throughout 1865, it was clear that Prussia intended to challenge the position of the Austrian Empire within the German Confederation. Despite potentially holding the balance of power between the two, Emperor Napoleon III kept France neutral. Although he, like most of Europe, expected an Austrian victory, he could not intervene on Austria's side as that would jeopardise France's relationship with Italy post-Risorgimento. As a result, at Biarritz on 4 October 1865, Napoleon III promised Prussian Prime Minister, Otto von Bismarck, France's neutrality, hoping that such an open statement of intent would strengthen France's negotiating position regarding the western bank of the Rhine. Bismarck refused to offer any land from the Rhineland, which was Napoleon's preferred region. However, he did make suggestions of French hegemony in Belgium and Luxembourg, although not committing anything to writing. When Austria and Prussia did go to war in 1866 (the so-called Seven Weeks' War), the result was a shock to Europe. Prussia quickly defeated Austria and her allies, forcing Austria to the negotiating table. Napoleon III offered to mediate, and the result, the Treaty of Prague, dissolved the German Confederation in favour of a Prussian-dominated organisation, the North German Confederation. French offer Assuming that Bismarck would honour his part of the agreement, the French government offered King William III