ethnic cleansing vs night of the long knives

ethnic cleansing

noun
  • The mass expulsion or killing of people belonging to one ethnic group by those of another. 

night of the long knives

noun
  • A purge, in which opponents of a regime or political party are killed or removed. 

How often have the words ethnic cleansing and night of the long knives occurred in a corpus of books? (source: Google Ngram Viewer )