would have to say the ongoing war/wars going on in both montreal and toronto from 2009-present day.
the amount of murders,attempted murders,arsons and general mayhem committed by the various factions and alliances is incredible.
now i realize that this may not be a single war like some of the other examples given but even short time periods r significant.

2009-2011
desjardins/dimaulo/devito faction vs. the rizzuto faction
-this 2 year war is crazy enough and what set off the ongoing violence.