"It can't happen here?" It's happened; it's happening.
The Second Amendment is Obsolete, some say. The idea that the United States could ever turn tyrannical is pure paranoia, some say.
Well, let's look at that.
Rounding up people and sending them to concentration camps (whether called "reservations" or "relocation centers"). Check. (Treatment of Native Americans. Japanese-American "Relocation Centers" during World War II).
Illegal medical experiments involving infecting people with diseases, not treating them, and observing the effects done on people without their knowledge or consent. Check. (Tuskegee Syphilis Experiment--and particularly interesting how that was "explained" to the victims as they were getting free health care from the US Government.)
Arbitrary searches of American citizens' households aimed at the seizure of property without either probable cause or any kind of warrant. Check. (post-Katrina gun Confiscation)
Laws passed allowing the indefinite detention of American Citizens without due process of law. Check. (NDAA 2012)
American citizens going about their daily business being stopped and searched again without probable cause or any kind of warrant (or even the "reasonably articulable suspicion" for a "Terry Stop"). Check. (TSA, not just at Airports, but at bus terminals, rail and subway terminals, highways, even High School Proms.)
"Can't happen here?" It has and is happening here.