How and why did Joseph Stalin (a “thug” and “enforcer”

  How and why did Joseph Stalin (a “thug” and “enforcer” for Lenin) rise to absolute power (and eventually killing his rival Leon Trotsky), killing millions while modernizing, industrializing, and doing the collectivization of the Soviet Union? 

4 pages long, 3 sources from any legitimate mainstream book, journal article, periodical, or newspaper (in that order of importance).  No dictionaries or encyclopedias, especially Wikipedia. cite a source using the APA Style for anything that is not common knowledge, let alone if you quote from it. Use standard formatting (double-spaced, 12-point Times or Times New Roman, normal margins, page numbering, etc.). Do not do a Title Page or sub-headings. Do a Works Cites section (which will not count for length).