It all started when the Supremes elevated "reasonable" over "warrant" in their interpretation of the 4th Amendment. Anyway, from this article at LewRockwell.com:
The author then examines several cases showing what has now become a rather rapid erosion of our 4th Amendment rights. Read the whole thing.
In the police state being erected around us, the police can probe, poke, pinch, taser, search, seize, strip and generally manhandle anyone they see fit in almost any circumstance, all with the general blessing of the courts. Making matters worse, however, police dogs – cute, furry, tail-wagging mascots with a badge – have now been elevated to the ranks of inerrant, infallible sanctimonious accusers with the power of the state behind them. This is largely due to the U.S. Supreme Court’s recent ruling in Florida v. Harris, in which a unanimous Court declared roadside stops to be Constitution-free zones where police may search our vehicles based upon a hunch and the presence of a frisky canine.
This is what one would call a slow death by a thousand cuts, only it’s the Fourth Amendment being inexorably bled to death. This latest wound, in which a unanimous Supreme Court determined that police officers may use drug-sniffing dogs to conduct warrantless searches of cars during routine traffic stops, comes on the heels of recent decisions by the Court that give police the green light to taser defenseless motorists, strip search non-violent suspects arrested for minor incidents, and break down people’s front doors without evidence that they have done anything wrong.
These are the hallmarks of the emerging American police state, where police officers, no longer mere servants of the people entrusted with keeping the peace, are part of an elite ruling class dependent on keeping the masses corralled, under control, and treated like suspects and enemies rather than citizens. Whether it’s police officers breaking through people’s front doors and shooting them dead in their homes or strip searching innocent motorists on the side of the road, in a police state such as ours, these instances of abuse are not condemned by the government. Rather, they are continually validated by a judicial system that kowtows to every police demand, no matter how unjust, no matter how in opposition to the Constitution.
The justices of the United States Supreme Court through their deference to police power, preference for security over freedom, and evisceration of our most basic rights for the sake of order and expediency have become the architects of the American police state.