The US Justice Department is suing the state of Arizona over SB 1070, because “The Individual Sections of S.B. 1070 are Prempted by Federal Law“. The theory, as I understand it, is that the Feds assert that only the Feds should be enforcing Federal Immigration law. The irony ( actually, one of many ) is that for the last 15 years, the Federal Homeland Security department has been running the ICE 287(g) program, in which Immigration and Customs Enforcement trains local police to enforce federal law. The program has 9 signed Memorandums with Arizona police departments. So, it appears that one federal department is suing Arizona for doing what another federal department is training them to do.