"Missouri's governor said on Monday he would send the National Guard into the St. Louis suburb of Ferguson to restore calm after authorities forcibly dispersed a crowd protesting last week's fatal shooting of an unarmed black teen by police.
Gov. Jay Nixon signed an executive order deploying the U.S. state militia, saying demonstrators had thrown Molotov cocktails and shot at police as well as a civilian, a description of the night's events diverging widely from some eyewitness accounts." Reuters
"Whenever an insurrection occurs in any State against its government, the President may, upon the request of its legislature or of its governor call into Federal service such of the militia of the other States. This is a statutory exception to the PCA." wiki on US National Guard.
I assume that the provision in law for the employment of the National Guard to suppress "insurrection" is the basis for the employment of the Missouri Army National Guard in Ferguson. I believe that this kind of employment for restoration of public order can be federally funded without regard to the Posse Comitatus Act (PCA). This is important because the National Guard will need the power of arrest in this situation and in most federal mobilization situations the National Guard does not have the power of arrest.
Will the introduction of the National Guard calm the city of Ferguson or will it escalate the disorder there? IMO that is a question yet to be answered. National Guardsmen will make good targets for rioters. Will the NG deploy with loaded weapons? what will be their rules of engagement? pl