Invasion of Poland (1939)
It is often called the first time that blitzkrieg was tried on the battlefield, but similar methods had been used earlier.  The German surprise attack was successful and was very effective against the ineffective and demobilized Polish Army, whose tanks and airplanes were few and mostly old.  The Poles were outflanked, outmaneuvered, and outnumbered in September 1939 and easily destroyed by the Germans.  The Poles, if they had been well prepared, could have had two million soldiers in the fight.