After the Franco-Prussian War of 1870, the area of Alsace-Lorraine was annexed by the newly created German Empire in 1871 by the Treaty of Frankfurt and became a "Reichsland". Thionville once again became "Diedenhofen" and became a prosperous city. Some large neo-Romanesque buildings typical of the German Empire were constructed in the city. The German Army decided to build a fortress line from Mulhouse to Luxembourg to protect the new "Reichsland". The centerpiece of this line was the great "Moselstellung", a fortress system protecting Metz and Diedenhofen.