WordNet (r) 3.0 (2006):

warlord
    n 1: supreme military leader exercising civil power in a region
         especially one accountable to nobody when the central
         government is weak

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "warlord":
   Simon Legree, absolute monarch, absolute ruler, all-powerful ruler,
   arrogator, autarch, autocrat, caesar, commissar, czar, despot,
   dictator, disciplinarian, driver, duce, hard master, martinet,
   oligarch, oppressor, pharaoh, slave driver, stickler, tyrant,
   usurper