Trump vows national emergency in Washington, DC over ICE dispute

[snot]

2. I'm extremely concerned about this whole approach

of deploying federal forces to address state crimes.

I.e., ok, immigraiton is a national concern (and states that are stuck with any costs for immigration law enforcement should accordingly be completely reimbursed by the federal government), so if that's all Trump were doing, I could at least see an argument that it might be legal.

But he's openly declaring that his actions are at least partly about crimes – murder, rape, theft, etc., all of which are matters of STATE law and enforcement; so i don't see how he has any authority at all to order federal forces to get involved in such things, especially when it's over the objections of local authorities.

Seriously, is any rationale being offered beyond "it's really terrible so someone's got to do something"?