foot soldier vs sapper

foot soldier

noun
  • A soldier who fights on foot; an infantryman 

  • A rank and file member of an institution, or supporter of a cause. 

sapper

noun
  • Historical term for a combat engineer that is still used in some armies, in other words an engineer or a soldier engaged in attacking, destroying, and circumventing or building fortifications, bridges, and roads; a military engineer active in a combat zone. 

  • An officer or private of the Royal Engineers. 

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