Who uses it: Constitutional lawyers and military historians
What it means: Literally, "the power of the county;" as a term of art, it refers to the longstanding common law practice (and the statutory Posse Comitatus Act) that prohibits American military units from acting as civil police.
How you can use it: When discussing the President's immigration policy.
The idea of posting National Guard troops along our border with Mexico strikes me as both dangerous and wasteful. It's not really a posse comitatus issue, because this is theoretically a question of national security; protecting the country from foreign invaders is what the National Guard is for. But the National Guard is already severely overtaxed, and the number of troops the President is talking about deploying will hardly make a difference to the government's presence along the border. As someone on the news noted last night, the number of troops works out to fewer than one Guardsman every 600 yards, even if each one works 24 hours a day.
More to the point, every effort to tighten the U.S. - Mexico border has just made people more creative and desperate, endangered more lives and put more money in the hands of the most unscrupulous smugglers and criminals. Why would this attempt be any different?
It's a long-term problem with no short-term solution. Maybe issuing meaningful work permits, and enforcing their use, will help -- anything that provides opportunities and incentives for entering the country legally is a good idea.
I'm off to Brookline this afternoon; from there to Norfolk for the CSBS Annual Meeting, and from there to Washington for BEA. Posts over the next few days will be erratic, but I'll try to check in once a day.