Controversy continues over Donald Trump's executive order that bans all refugees and refuses entry to immigrants and non-immigrants from seven Muslim-majority countries. The Department of Justice head stated that the DoJ will not defend the order, and acting Attorney General Sally Yates' letter about Trump's immigration order gives insight into exactly why.
The letter states that Yates believes it is her responsibility to make sure the Justice Department's position on the executive order is legally defensible and is informed by "our best view of what the law is after consideration of all the facts." While she said that the Office of Legal Counsel reviews an order to see if its lawful on it's face, it does not take into context what the administration or its surrogates have previously said. That, she believes, is where the Department of Justice comes in.
If you'd like it summed up, basically the letter says that Yates is taking into consideration everything Trump has said about Muslims, immigration, refugees, etc., and is using that as context to determine the legality of the executive order he issued temporarily banning refugees and immigrants — and she doesn't think it's going to pan out as "legal," so her department won't defend it. But if you'd like to read the letter in full to see exactly what she said, here it is in full.