When Nazi Germany attacked the Soviet Union on June 22, 1941 during the course of Operation Barbarossa, most of the line was not yet finished, and hence posed a negligible obstacle to the invading forces. Only the four southernmost regions, partly completed, were able to hamper the advance of the Wehrmacht for a few days. (The Brest Fortress resisted much longer, but it was an older fortification technically not part of the Molotov line).