Fortress

Before 1914 fortresses were meant to bar invasion routes and tie down enemy forces. They were built as rings of concrete forts a few kilometres outside a city, like Brialmont's Belgian forts around Liège and Namur, Austrian Przemyśl, or the Russian fortresses in the Kingdom of Poland, Novogeorgievsk (Modlin) and Ivangorod (Dęblin). The war showed that outdated forts could not withstand the heaviest mortars: the forts of Liège fell within a fortnight in August 1914. A fortress could also be beaten by hunger: Przemyśl, besieged from November 1914, surrendered on 22 March 1915.

In summer 1915 the Germans and Austrians took Ivangorod and Kovno, the Russians abandoned Osowiec, and Novogeorgievsk surrendered after a short siege together with tens of thousands of men. From then on fortresses lost importance to continuous trench lines, although at Verdun in 1916 old forts such as Douaumont still played an important part.