• [What allowed the Germans to defeat the Maginot Line?] (http://www.reddit.com/r/AskHistorians/comments/2onrnk/after_world_war_i_france_was_the_most_powerful/cmoxmga?context=3)
• [What if France did not build the Maginot Line?] (https://www.reddit.com/r/AskHistorians/comments/2zijiq/if_the_french_had_not_built_the_maginot_line/)
• [Why did the Siegfried Line work, unlike the Maginot Line?] (https://www.reddit.com/r/AskHistorians/comments/3412u1/in_wwii_why_did_the_siegfried_line_work_when_the/)
• [The Myth of Blitzkrieg] (https://www.reddit.com/r/AskHistorians/comments/3nq7y4/what_tacticsvehicle_designsideas_did_the_germans/)
• [German armoured warfare developments prior to WWII] (https://www.reddit.com/r/AskHistorians/comments/3khm62/generally_our_ancestors_always_waged_war_in_the/cuyc1xy)

These previous answers I've given should be pertinent

The French didn't extend the Maginot Line along the Belgian Border because A) the water table is high in Flanders, and the soil is soft, which would have made constructing defences difficult; B) the French didn't want to give the impression that they would abandon the Belgians; and C) the French were spending money on tanks, aircraft, their Navy, and the late 20s and the 30s were bad economic times, they couldn't shell out more funds to essentially double the length of the line.

The whole concept of the Maginot Line was predicated on the Germans repeating an invasion of Belgium! That way Britain, France and Belgium would start any future war together, rather than separate. It would keep the war away from France, and would enable the French to predict and plan for a German offensive in advance, staking out likely routes of invasion via Belgium. As I mention in the answers above, it was the loss of air superiority and the lack of a strategic reserve, combined with poor command and communications that did the Allies in in 1940, and even then, the French fought on tenaciously after Dunkirk.