Envelopment is the military tactic of surrounding an enemy in the field so that they are isolated in a pocket. The friendly forces can choose to attack the pocket or invest

Investment (military)

Investment is the military tactic of surrounding an enemy fort with armed forces to prevent entry or escape.A circumvallation is a line of fortifications, built by the attackers around the besieged fortification facing towards the enemy fort...

 it (to stop supplies getting and to prevent breakouts) and wait for a beleaguered enemy to surrender.

To achieve an envelopment several different tactics can be employed:

• A flanking manoeuvre can envelop an enemy force if some form of obstacle (such as a river, sea, or a mountain range) prevents the enemy from retreating or obtaining supplies.
• A pincer movement
  Pincer movement
  The pincer movement or double envelopment is a military maneuver. The flanks of the opponent are attacked simultaneously in a pinching motion after the opponent has advanced towards the center of an army which is responding by moving its outside forces to the enemy's flanks, in order to surround it...
  
   consists of two simultaneous flanking manoeuvres which encircle an enemy. Early in World War II
  World War II
  World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
  
   the Germans frequently employed this tactic and encircled huge numbers of the enemy during the Blitzkrieg
  Blitzkrieg
  For other uses of the word, see: Blitzkrieg Blitzkrieg is an anglicized word describing all-motorised force concentration of tanks, infantry, artillery, combat engineers and air power, concentrating overwhelming force at high speed to break through enemy lines, and, once the lines are broken,...
  
  attacks on both the Western Front during the Battle of France
  Battle of France
  In the Second World War, the Battle of France was the German invasion of France and the Low Countries, beginning on 10 May 1940, which ended the Phoney War. The battle consisted of two main operations. In the first, Fall Gelb , German armoured units pushed through the Ardennes, to cut off and...
  
   and during Operation Barbarossa
  Operation Barbarossa
  Operation Barbarossa was the code name for Germany's invasion of the Soviet Union during World War II that began on 22 June 1941. Over 4.5 million troops of the Axis powers invaded the USSR along a front., the largest invasion in the history of warfare...
  
   on the Eastern Front
  Eastern Front (World War II)
  The Eastern Front of World War II was a theatre of World War II between the European Axis powers and co-belligerent Finland against the Soviet Union, Poland, and some other Allies which encompassed Northern, Southern and Eastern Europe from 22 June 1941 to 9 May 1945...
  
  .
• A retreat where the enemy advances and then is attacked by friendly forces in the rear
  Rear (military)
  In military parlance, the rear is the part of concentration of military forces that is farthest from the enemy . The rear typically contains all elements of the force necessary to support combat forces - food, medical supplies and substantial shelters, planners and command headquarters....
  
   as happened at the Battle of Cannae
  Battle of Cannae
  The Battle of Cannae was a major battle of the Second Punic War, which took place on August 2, 216 BC near the town of Cannae in Apulia in southeast Italy. The army of Carthage under Hannibal decisively defeated a numerically superior army of the Roman Republic under command of the consuls Lucius...
  
  .
• A vertical envelopment is "a tactical maneuver in which troops, either air-dropped or air-landed, attack the rear and flanks of a force, in effect cutting off or encircling the force".