Short answers, pulled from the story.
Construction began in 1930 under the Service Technique du Génie overseen by the Commission d'Organisation des Régions Fortifiées. The main construction was largely completed by 1939 at a cost of around 3 billion French francs.
French leaders believed they needed Britain's help to win another war against Germany because they had no hope of defeating Germany on its own from 1871. By 1929, the decision to build the line became a tacit admission that Germany would soon rearm while the French economy remained hobbled by World War I reconstruction needs.
There are 45 main forts known as grands ouvrages and 97 smaller forts called petits ouvrage. These structures connect over 200 kilometers of tunnels with infrastructure including power-generating units and water storage systems.
On the 10th of May 1940 German forces were well into France within five days after violating neutrality of Belgium Luxembourg and Netherlands. They crossed the Meuse river and raced down the Somme valley encircling much of Allied forces in the north leading to the Dunkirk evacuation.
On the 19th of May 1940 the German 16th Army captured isolated petit ouvrage La Ferté after conducting deliberate assault by combat engineers backed up heavy artillery. The entire French crew of 107 soldiers was killed during action taking fortifications in only four days.