From 17 December 1926 to 12 October 1927, the Frontier Defence Commission reported to the CSG that fortifications should be built from Metz to Thionville and Longwy, to protect the Moselle Valley and the mineral resources and industry of Lorraine. The area around the River Lauter, the most north-eastern part of the common border with Germany, should be fortified as an obvious invasion route but there was no need to fortify the Rhine, because of the Vosges mountains further west and the small number of railways on the German side. Belfort was near the Swiss frontier and partly protected by the Rhine but there was an avenue of invasion to the west, which should be protected. The commission gave emphasis to defence against a surprise attack, with the limited objective of capturing the Metz and Lauter areas. The commission recommended that priority be given to protecting the resources and industries of Lorraine that were vital for the French economy and would become more important for a war economy. The nature of fixed defences was debated during the 1920s, with advocates of the offensive use of fortifications, deep or shallow defences and centralised and decentralised designs. On 12 October 1927, the CSG adopted the system recommended by Pétain, of large and elaborately fortified defences from Metz to Thionville and Longwy, at Lauter and Belfort, on the north-east frontier, with covered infantry positions between the main fortifications. The Minister of War André Maginot (1922–1924, 1929–1930 and 1931–1932) became the driving force for obtaining the money to fortify the north-eastern frontier, sufficient to resist a German invasion for three weeks, to give time for the French army to mobilise. Work began in 1929 on the (Metz Fortified Region) through the Moselle valley to the Nied at Teting, then the , east of Hagenau from Bitche to the Rhine, the extension of the Metz region to Longuyon and the Lauter region from Bitche to the Sarre at Wittring. The requirements for the fortifications were natural cover, sites nearby for observation posts, the minimum of dead ground, a maximum arc of fire, ground suitable for anti-tank obstacles and infantry positions and ground on which paved roads could be built, to eliminate wheel marks. were to be built near the frontier as permanently garrisoned works, whose men would alert the army, blow bridges and erect roadblocks, for which materials were dumped.