Immigration is an American Value

On its face it seems so sane and simple: a new Arizona law that "requires local police to enforce federal immigration regulations." What is wrong with that? Enforcing existing regulations is a good thing, right? Except that it is not for the states to enforce federal regulations. So what are the federal regulations in question? What is the purpose of immigration policy? And what is the issue Arizona is actually trying to address with its state law?

I have spent no small amount of time over the last few weeks trying to wrap my head around these questions....