 Artillery was coordinated with protective measures to ensure that one fort could support the next in line by bombarding it directly without harm.The largest guns were therefore fortress guns; larger weapons were to be part of the mobile forces and were to be deployed behind the lines.The fortifications did not extend through the Ardennes Forest (which was believed to be impenetrable by Commander-in-Chief Maurice Gamelin) or along France's border with Belgium, because the two countries had signed an alliance in 1920, by which the French army would operate in Belgium if the German forces invaded.