Iraq, not Afghanistan; and it wasn't an ambulance—it was private vehicle with an uninvolved civilian, and two children. You can read one account of it here (from the PoV of one of the soldiers):
We can't be a "beacon of democracy" so long as we're shooting children from helicopters, covering it up as "national security", and prosecuting journalists who disclose these things to the US public. Democracy is premised on its people understanding the things its government is doing in their name.
