I'm not surprised that a cop(or anyone) enters a yard and the dog goes on the offensive, that's why we have dogs. No contact with the home owner before letting himself in.
Help yourself to my front door and I might go on the offensive too.
If you are meaning a police officer letting themselves in then you would be in the wrong.
"My message to my troops is if you see anybody carrying a gun on the streets of Milwaukee, we'll put them on the ground, take the gun away and then decide whether you have a right to carry it." - Milwaukee Police Chief Ed Flynn
