text: Offense
  of a tactical, operational, or strategic mission (JP 3-18). An air assault operation is an operation in which
  assault forces,using the mobility of rotary-wing or tiltrotor aircraft and the total integration of available fires,
  maneuver under the control of a ground or air maneuver commander to engage enemy forces or to seize and
  hold key terrain (JP 3-18). An enemy may or may not be in a position to oppose the maneuver. While the
  commander should attempt to achieve an unopposed landing when conducting a vertical envelopment, the
  assault force must prepare for the presence of opposition. (SeeFM 3-99 for additional information.)
  Figure 6-3. Vertical envelopment (example air assault)
  ENCIRCLEMENT
  6-99. Encirclement is a result of envelopment where a force loses its freedom of maneuver because an
  opposing force is able to isolate it by controlling all ground lines of communications and reinforcement. The
  commander conducts offensive encirclements to isolate an enemy force. Typically,encirclements result from
  penetrations and envelopments, or are extensions of exploitation and pursuit operations. As such,
  encirclements are not a separate form of offensive operations but an extension of an ongoing operation.
  Encirclements may be planned sequels or result from exploiting an unforeseen opportunity. Encirclements
  usually result from the linkup of two encircling arms conducting a double envelopment. However,
  encirclements can occur in situations where the attacking commander uses a major obstacle, such as a
  shoreline, as a second encircling force. Although a commander may designate terrain objectives in an
  encirclement,isolating and defeating enemy forces are the primary goals. Ideally,an encirclement results in
  the surrender of the encircled force. This minimizes friendly force losses and resource expenditures.