Why was Luxembourg’s fortress dismantled after the Treaty of London in 1867?

The dismantling of Luxembourg’s fortress was a direct result of the compromise reached by the great powers at the Treaty of London on May 11, 1867.12

Following the dissolution of the German Confederation in 1866, Luxembourg became a sovereign nation, yet a Prussian garrison remained stationed in its capital. Tensions escalated when Napoleon III of France attempted to purchase the grand duchy from William III, a move strongly opposed by the Prussian chancellor, Otto von Bismarck.123

To resolve the crisis and avoid conflict, the great powers agreed to a settlement that required Prussia to withdraw its garrison from the capital and mandated that the fortress—long considered the strongest in Europe after Gibraltar—be dismantled. As part of this agreement, Luxembourg was declared an independent, perpetually neutral state. The process of dismantling the extensive fortifications, which included 15 miles of casemates, took 16 years to complete.413