Arizona Sheriff Joe Arpaio is well-known for his unconventional style of law enforcement, for being a birther and for his "posse," a group of unofficial volunteer crime fighters.
The Department of Justice has a problem with that last one. The DOJ attempted to negotiate with Arpaio on he and his posse's aggressive crackdowns on illegal immigration back in February. An agreement was drafted and issued to Arpaio that would severely limit the authority of his posse.
The posse would essentially have to stay away from enforcing immigration policy altogether, according to the draft made public by Talking Points Memo.
[T]he draft would have banned the Maricopa County Sheriff’s Office from using posses in the human smuggling unit, in its criminal employment squad and during immigration-related criminal suppression operations. ...
Arpaio would have been required to develop standards for the posse members and would have to discontinue operation of his immigration posses, prohibit the participation of posse members in crime suppression operations, workplace raids and immigration-related law enforcement activities.
In short, Arpaio's posses would just be there to investigate more birth certificates.
The DOJ has indicated that it will go through with a lawsuit, claiming Arpaio has not followed through with the agreement.