Maginot Line
Had the French finished the line the German tanks and troops would not have been able to get across without great cost. Since they did not finish it the Germans just rolled through the northern opening.
The Maginot line was a classic blunder in warfare history. Even Napolean said, “the army that stays in its fort is doomed”. The French did not listen to this wisdom. Even if it were finished, the Germans would have gone a different direction around it, or sent Airborne troops behind it. The lesson is to be mobile, agile, and engage the enemy on your terms...