What tactics did germany use in france, forcing a desperate british withdrawal at dunkirk?

The Blitzkrieg. This was Utilized by Invading Belgium and Sending Tanks through the Argonne Forest. The encirclement of the Maginot was also an important Tactic/Decision which led to the Surrender of the Majority of the French Army and forced the British to retreat to Dunkirk and Evacuate over the Channel.