As I wrote recently in another thread, one of the main things that's driving the militarization of police and sheriff's departments nationwide is the fact that they get most of the equipment free, courtesy of the federal government. Since the 1990s, the Departments of Defense and Homeland Security have been donating surplus military equipment to local law enforcement agencies. The donated equipment includes rifles and handguns, grenade launchers, helicopters, armored personnel carriers -- pretty much anything you might think of, in fact.
Here's an
article describing some of the goodies that have gone to local departments in just one state (Georgia).
It stands to reason that once they have the military stuff, their tactics, training, and policies are influenced by what's available.
It's worth asking why the federal government is pushing the militarization of police by providing this equipment. IMO, it's the WoTerror, as well as the WoDrugs... at least indirectly.
And deepcreek is correct that criminalizing Americans has become big business, as well.