Mexico's new president, Felipe Calderon, seems to be doing all the right things in cracking down on Mexico's drug traffickers. He's appointed new people to key military and criminal justice positions, deployed troops to quell drug violence, reasserted federal police power, extradited a few major traffickers to the United States, and given a green light to legislation clarifying Mexico's drug laws. But all this provides little reason to hope that Mexico has really turned a corner in its efforts to control the illegal drug trade. For a guide to what's in store, one need only look at past sexenios (the six year terms of Mexican presidents).
Past Mexican presidents have been unsuccessful in fighting the drug trade
From the context, it's obvious that what past presidents have done hasn't worked to fight the drug trade. I think it was difficult for the system because it's a fairly generalized statement. 
( z0 succeed :ARG0 ( z1 person :ARG0-of ( z2 have-org-role :ARG1 ( z3 country :name ( z4 name :op "Mexico" ) ) :ARG2 ( z5 president ) :time ( z6 past ) ) ) :ARG1 ( z7 fight :ARG0 z1 :ARG1 ( z8 trade :ARG1 ( z9 drug ) ) ) :polarity - )
(z0 / succeed-01
    :ARG0 (z1 / person
              :ARG0-of (z2 / have-org-role-91
                           :ARG1 (z3 / country
                                     :name (z4 / name
                                               :op1 "Mexico"))
                           :ARG2 (z5 / president)
                           :time (z6 / past)))
    :ARG1 (z7 / fight-01
              :ARG0 z1
              :ARG1 (z8 / trade-01
                        :ARG1 (z9 / drug)))
    :polarity -)
Past Mexican presidents have been unsuccessful in fighting the drug trade
z3 :name z4
z4 :op1 1
z1 :ARG0-of z2
z2 :ARG1 z3
z2 :time z6
z2 :ARG2 z5
z0 :polarity 0
z0 :ARG0 z1
z0 :ARG1 z7
z7 :ARG0 z1
z8 :ARG1 z9
z7 :ARG1 z8