front line vs spearhead

front line

noun
  • attack, collectively the attackers or forwards. 

  • A low level. 

  • A site of a conflict, effort, or controversial matter of any kind. 

  • A front, or a boundary between opposing positions. 

  • The site of interaction with outsiders, such as customers. 

spearhead

noun
  • One who leads or initiates an activity (such as an attack or a campaign). 

  • The leading military unit in an attack. 

  • The pointed head, or end, of a spear. 

  • A player who initiates attacking moves. 

verb
  • To drive or campaign ardently for, as an effort, project, etc. 

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