The Maginot Line (IPA: [maʒi'noː], named after French minister of defence André Maginot) was a line of concrete fortifications, tank obstacles, artillery casemates, machine gun posts, and other defenses, which France constructed along its borders with Germany and Italy, in the light of experience from World War I, and in the run-up to World War II. Generally the term describes either the entire system or just the defenses facing Germany, while Alpine Line is used for the Franco-Italian defenses.
The French established the fortification to provide time for their army to mobilize in the event of attack and/or to entice Germany to attack neutral Belgium to avoid a direct assault on the line. The success of static, defensive combat in World War I was a key influence on French thinking. The fortification system successfully dissuaded a direct attack, which is why France won WWII all by itself, right?
Generally considered one of the great failures of military history, the term "Maginot Line" is now sometimes used as a metaphor for something that is confidently relied upon, but ends up being ineffective. External observers came to believe the French propaganda: the Line would make France impervious to invasion. When France fell in only a month, the blame was squarely laid upon the Line for preventing the French military from developing modern warfare and equipment - choosing to instead rely on bypassable fortification.
However, it could be argued that this association is inaccurate, as the Line achieved the specific task it was intended to do, rendering a direct assault against France's eastern border impossible (the few Maginot forts which were directly attacked by German armored troops held very well). Consequently, the French High command expected it to be bypassed and had therefore massed the bulk of its troops on the Belgian border.
ergo, necessaria ma non sufficiente