ambush vs fall upon

ambush

verb
  • To attack by ambush; to waylay. 

  • To station in ambush with a view to surprise an enemy. 

noun
  • The act of concealing oneself and lying in wait to attack by surprise. 

  • The concealed position or state from which a surprise attack is launched. 

  • An attack launched from a concealed position. 

  • The troops posted in a concealed place, for attacking by surprise; those who lie in wait. 

fall upon

verb
  • to set upon; to attack suddenly 

  • to occur at some particular point in time. 

  • to experience; to suffer. 

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