lieutenant commander vs warlord

lieutenant commander

noun
  • A naval officer whose rank is higher than a lieutenant and below that of a commander. 

warlord

noun
  • A high military officer in a warlike nation. 

  • A local ruler or bandit leader usually where the government is weak. 

How often have the words lieutenant commander and warlord occurred in a corpus of books? (source: Google Ngram Viewer )