Nazi Germany created the the largest, heaviest artillery piece ever used in combat. Here's What You Need To Remember: Due to complications in building such a massive gun, the Schwerer Gustav was not used against the Maginot Line, though it did see action against the Red Army. Leading up to the Second World War, one of the obstacles that loomed large in the eyes of the German High Command was the French Maginot Line. The Line was one of the strongest defensive fortifications ever built and among the longest: it stretched from the French-Swiss-German border in the South, all the way to the French side of the English Channel to the North.