This article is from the FAQ, by with numerous contributions by others.
(a) If you advocate or if you are a member of any organization
that is opposed to organized government (i.e., if you preach
and practice anarchy).
(b) Membership in communist organizations.
(c) If you advocate the overthrow of the U.S. Government by force,
sabotage, violence or terrorism.
(d) If you publish any material advocating the methods of item (c).
(e) Exemption from services in the armed forces of the U.S.
(unless the alien status does not permit the individual to
serve on the armed forces or if the alien had served in the
armed forces of his/her own country).
(f) Desertion from military forces and draft evasion results in
permanent ineligibility for citizenship.