Among other things, yes. That was defined when the Department of Defense was established and has been further defined since then.
And is that SecDef required to be appointed by the President, according to the Constitution?
Indeed he is. Title 10 > Subtitle A > Part I > Chapter 2 > § 113 of the U.S. code. But the Secretary of Defense is appointed by the president, with the advice and consent of the Senate. What about it?
Remember that manual that One Winged Shark quoted from, where the “filling sandbags” quote even came from - saying that all orders right down to filling sandbags are the execution of the entire chain of command, stemming from the authority of the Constitution?
I do.
How do you think that manual came up with that? Do you think it has anything to do with the laws that Congress has passed regarding who authorizes what in the military?
Without knowing exactly what book he was quoting from it's hard to say. But I would point out that whatever book it was, it said the President's authority extends through all chains of command. It did not say that all commands derive their authority from the commander-in-chief.
The sad thing about this is that nobody with any comprehension of the law expected The U.S. Army Court of Criminal Appeals to do anything but summarily deny this writ. For simply pointing out that obvious fact, we were subjected to the usual ravings and tedious, sophmoric attempts at wit. And, as happens every single time, when the obvious unfolded exactly as predicted, there's never a shred of actual thought as to why that might be. The U.S. Army Court of Criminal Appeals simply joins the ever-expanding list of “traitors.”