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. However, after France had failed to counter the German remilitarisation of the Rhineland, Belgium—thinking that France was not a reliable ally—abrogated the treaty in 1936 and declared neutrality. France quickly extended the Maginot Line along the Franco-Belgian border, but not to the standard of the rest of the line. As the water table in this region is high, there was the danger of underground passages getting flooded, which the designers of the line knew would be difficult and expensive to overcome.