air force vs militia

air force

noun
  • A unit in the U.S. Air Force larger than a division but smaller than a command. 

  • A branch of the military mainly devoted to air warfare, as opposed to (land) army and navy. 

militia

noun
  • An army of trained civilians, which may be an official reserve army, called upon in time of need, the entire able-bodied population of a state which may also be called upon, or a private force not under government control. 

  • Synonym of militsia: the national police force of certain countries (e.g. Belarus). 

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