encirclement vs envelopment

encirclement

noun
  • The act of encircling or the state of being encircled 

  • The isolation of a target by the formation of a blockade around it 

envelopment

noun
  • The act of enveloping. 

  • An offensive action in which an attacking force moves over or around the enemy and attacks from the rear; see also pincer movement. 

  • That which envelops; a covering. 

  • An action to seize the opponent's blade in one line and lead it (without losing contact) through a full circle to end in the same line. 

How often have the words encirclement and envelopment occurred in a corpus of books? (source: Google Ngram Viewer )