Ben Cardin actually said...

We are now the only country that has announced that we will not be part of these discussions.

So there is discussion taking place at the National Security Council level with legal advisers, with military advisers.

there is not a military solution to the crisis in North Korea, that any military option carries unbelievable risk factors

You may very well be getting opinions from the White House that this is legal, but you would have to make your own independent judgment.