The word "posse" is a shortened form of the Latinate legal phrase posse comitatus. The "posse" half is a contracted form of potis esse, an adjective-infinitive pair meaning "to be able." In Latin, it stood on its own as the present infinitive form of the irregular verb possum, posse, potui ("I am able, to be able, I have been able"), from which we also derive "possible" and "potent" (by way of the present participle potens). The "comitatus" half is the perfect participle of the verb comito, comitare, comitavi, comitatus ("I accompany, to accompany, I have accompanied, having been accompanied"). Both these senses -- power and joining together -- are present in the definition technical definition of the term given by Black's Law Dictionary (following Blackstone) as "The power or force of the country. The entire population of a country above the age of fifteen, which a sheriff may summon to his assistance in certain cases; as to aid him in keeping the peace, in pursuing and arresting felons, etc."

That is, a "posse" might be reasonably understood as a non-military militia, a group of civilians to be called out by law enforcement in times of need. In 1878, Congress passed the Posse Comitatus Act, the relevant portion of which read:

From and after the passage of this act it shall not be lawful to employ any part of the Army of the United States, as a posse comitatus, or otherwise, for the purpose of executing the laws, except in such cases and under such circumstances as such employment of said force may be expressly authorized by the Constitution or by act of Congress; and no money appropriated by this act shall be used to pay any of the expenses incurred in the employment of any troops in violation of this section And any person willfully violating the provisions of this section shall be deemed guilty of a misdemeanor and on conviction thereof shall be punished by fine not exceeding ten thousand dollars or imprisonment not exceeding two years or by both such fine and imprisonment.

Historically, the Act was part of the dismantling of Reconstruction. The victorious North had used military government in the South to advance Reconstruction, and the whites of the South, eager for a return to the days of legally-enforced racism, was desparate to get the Army out of the law-enforcement business once and for all. The Posse Comitatus Act is best seen as part and parcel of the Democrats' price for letting the Republicans steal the 1876 election.

The act has been variously amended, and the relevant portion now reads

Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

Leaving aside the oddities that the Navy and Marines appear to be exempt from the Act and that a ten-thousand dollar fine means somewhat less now than it did in 1878, the amendments have preserved the key central language prohibiting using the military "as a posse comitatus or otherwise to execute the laws." This phrase is a little odd, for two reasons. The first is that a standing army -- as opposed to the militia -- would appear to be incapable of serving "as a posse comitatus", since the definition of a posse makes it clear that it consists of the "entire population." And second, there's that seemingly redundant "otherwise" clause. Blackstone's definition is non-exclusive; he refers only to "certain cases," and then provides examples. "Pursuing and arresting felons" may be a duty of a posse, but nothing in the definition requires that a posse not carry out other duties for the "assistance" of the sheriff. The "otherwise" clause only makes sense if there are certain duties which consist in "enforcing the laws" but which are either not for "his assistance" or not carried out by "the entire population." The former possibility is absurd (at least unless one wants to argue fine technicalities about delegation of powers within the executive branch), which leaves the latter. But in this case, it is the first clause that's redundant, and the Act should simply prohibit the Army from "enforcing the laws" and drop that pesky "otherwise" and have done with it without needing to even mention a posse comitatus.

My point, in this exegesis, is simply that the Posse Comitatus Act is perhaps not the clearest or most definitively stated section of the U.S. Code. It was part of a thoroughly political compromise between a bunch of out-and-out villains and a bunch of self-interested politicians; its sentiments are questionable and its language uncertain. And it is also one of the militia movement's best-beloved sections of the Code.

One organization inspired by the Act is the Sheriff's Posse Comitatus (investigated by the FBI twice; the relevant files have been released under the Freedom of Information Act). The SPC is a white supremacist organization, proudly displaying the Act on its home page. We are the real posse, is the message: law enforcement and the standing armed forces (along with the IRS and the other usual targets) are illegitimate and have no place. Latching onto the language forbidding the military from assisting federal law enforcement -- shades of Waco -- and laying claim to the mass populism of gatherings of citizens are both standards of freelance militia groups, but there's something about the whole posse comitatus concept that rings strangely here. This is a posse with no sheriff, a posse dedicated, in point of fact, to resisting the sheriff, rather than assisting him. Yes, they mention their "duly-elected Sheriff's," but still, something is askew about the enterprise. It's an odd inversion of a persecution complex: one starts to see allies in the strangest places. Much as with its misappropriation of the anti-slavery Union anthem "Battle Hymn of the Republic," the SPC is on strange ground as it clings to the Posse Comitatus Act, another relic of the Civil War. In the words of Inigo Montoya, "You keep using that word. I do not think it means what you think it means."