The Belfort Gap acquired renewed significance in the series of Franco-German wars in the nineteenth and early twentieth centuries. During the Franco-Prussian War, the Prussian army invaded France via the Wissembourg Gap and then swept down the length of Alsace and laid siege to Belfort, hoping to seize the city and then rush on through the Belfort Gap into central France. In the event the siege ended up dragging on for months, and an armistice was signed before it could be brought to a decisive conclusion. The wider war had delivered a conclusive result, however, and under the resulting Treaty of Versailles (1871) Alsace and Lorraine were ceded by France to the newly-established German Empire. The language border was used in 1871 to determine the demarcation line between Alsace and Burgundy, and France was thereby able to retain Belfort. It had however become a border town again, and a series of fortifications was therefore thrown up to protect it from any future German attack. At the outbreak of the First World War, French troops invaded Germany through the Belfort Gap, leading to the bloody Battles of the Frontiers. The most recent military advance through the Belfort Gap was that of the French I Corps in November 1944, under General De Lattre.