That might make sense, but you said you wanted to arm both sides, including the leftists.
If they don't have arms, they can't fight; if they can't fight, the governement forces can only arrest, wrist-slap, and release them.
US, EU, & UN wouldn't allow anything else. Hell, it is hard enough to 'get permission' to even shoot those who ARE shooting at you: for evidence, see Israel.
The Mexican government is so disgustingly corrupt, an all-oput revolution and then starting from scratch might be a good thing.