Although there were fewer than 200 soldiers in Pizarro's group, they defeated an empire of about 12,000,000.
Is the dependent clause adjectival or adverbial.
Adjective clauses start with these words: which, who, whose, whom, that.
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/adj.htm
Adverb clauses start with any of a whole raft of subordinating conjunctions:
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/advcls.htm