The moment Ferguson, Missouri, police officer Darren Wilson shot an unarmed teenager, a 25-year old Supreme Court case became the prism through which his actions will be legally judged.  To most people, an 18-year old unarmed man may not appear to pose a deadly threat.  But a police officer’s perspective is different.  And that is how an officer should be judged after the fact, Chief Justice William Rehnquist wrote in the 1989 opinion.  The Supreme Court case, decided at a time when violence against police was on the rise, has shaped the national legal standards that govern when police officers are justified in using force.  The key question about Wilson’s deadly August 9^th shooting of Michael Brown is whether a reasonable officer with a similar background would have responded the same way.  Meanwhile, the streets of Ferguson have been peaceful for another night, as protests and tensions have been subsiding in the Saint Louis suburb.  Governor Jay Nixon ordered the Missouri National Guard, which arrived Monday, to begin withdrawing.  He said he feels there’s been progress.