how can the Federal government sue Az?

The federal government contends that Arizona is violating the Due Process clause of the 14th amendment. This clause basically says that no state can enforce laws which infringe on the rights of other states or the powers specifically enumerated to the federal government. Since the power to secure the borders is specifically granted to the federal government, Arizona cannot make laws which infringe on that right.

"Senate Bill 1070 was passed by the Legislature and signed into law by Brewer in April. It made it a state crime to be in the country illegally and stated that an officer engaged in a lawful stop, detention or arrest shall, when practicable, ask about a person's legal status when reasonable suspicion exists that the person is in the U.S. illegally.

Brewer had tasked the Arizona Police Officer Standards and Training board with creating a training video for law enforcement around the state to help them comply with the law. Brewer gave AzPOST a month to develop the statewide training for law enforcement, and police across the state had a month to implement the training, which included a two-hour video and additional materials police departments used to supplement."

The law enforcement agencies who where qualified to enforce the Federal law you were talking about, were specially trained by the Federal gov't according to what the law permitted, not by some state training board.

because Az is a state, they cant make nor enforce federal law without the cooperation of the fed

otherwise we would have 50 different policies and the federal govt unable to enforce because everyone was doing something different

Obama doesn't want border security to be a national issue because of how little he's done, so he decided to sue to kill off the border issue for another year.

The feds can do anything they want to, and they have all the crooked lawyers and paid for liberal judges to swing everything in their favor..